$20

Get vegetables and make stir fry. Or you could cook up that chicken and make chicken salad for sandwiches again only needing veggies. Or else get an onion some beans and make chicken chili. All these things have recipes on the internet you can pick what you like. Also this Webmd article was a good help to me....

http://www.webmd.com/food-recipes/cheap-healthy-15-nutritious-foods-about-2-dollars

Rice and beans are both cheap and provide (respectively) carbohydrate and protein with fiber.
Peanut butter makes for a good, cheap source of fat.
Fruit and vegetables can be relatively expensive (per calorie) but do not shy away from frozen or canned versions.

I'm based in the UK so your prices may be different but per gram of protein the cheapest sources tend to be Cottage Cheese, Eggs, Tinned fish (especially tuna) and Protein Powders.

I would suggest Pizza, Quinoa Pizza

Take a microwave safe bowl and layer it with pizza sauce add boiled quinoa and grated cheese on top of it. Add chopped veggies and again layer it with cheese and veggies.
Microwave it for 2 minutes.

http://www.fabhow.com/make-quinoa-pizza.html

Canned or frozen veggies are much cheaper than fresh (same with fruit). Drumsticks and thighs tend to be the cheapest chicken parts.
And of course, the broke college student's standby: ramen. Ramen is very cheap.

Buy a whole chicken it can be used for many different things. Frozen vegetables are always good. Rice and beans can be used many different ways. Buy the large thing of oatmeal not individual packets and it lasts a long time. Get the paper on Wednesday or Sunday for the ads a see where the best prices are (Walmart always price matches). Do you have $20 for a week or only $20?

Tuna, eggs, head of lettuce, in season or frozen vegetables, oatmeal.

I was gonna say beans and rice too, make sure to pick up an onion and a green pepper (green tends to be the cheapest) with those you can make some awesome beans and rice for less than $1 a serving. Plus onions and peppers go in so many other things Here's the recipe I use. http://www.skinnytaste.com/arroz-congri-cuban-rice-and-black-beans/

Oatmeal or grits. That way you either eat it savory for lunch or sweet for breakfast. Personally, I'd get the grits...super cheap and you can add topping like you would for oatmeal or throw a bit of parmesan in there ( I like parm since it's a stronger cheese and a little goes a long way) or nutritional yeast or whatever you have in there.

RAMEN - this needs no explanation.

EGGS.. reasonably cheap protein. You can make frittatas, poach it and put it on your grits or crack an egg in your ramen right when you take it off the heat. Swirl around and it's like egg drop soup.

Cheap greens - get a bulk bag of spinach or whatever is on sale this week. I like spinach the best, though. You can sautee, add to omelets, shred and put in your ramen, eat raw for salad.

Frozen veggies. On sale you should be able to get a bag of frozen veggies for $1 each.

chicken legs - Not sure where you are, but I bought some here last weekend for $.79 a lb. If you can't get them under $.99 a pound, get a whole chicken and roast it. You'll be eating on it all week.

Potatoes - very filling and usually $2-4 for a 5 pound bag in my area.
Apples - buying a bag instead of loose seems to be cheaper here. Also seems to be cheaper that other fresh fruits.
On sale, store brand frozen veggies.
Onion/garlic/ginger - usually cheap, goes a long way, and adds flavor
Rice
Eggs
Peanut butter
Bulk bin spices instead of jars - buy only what you need for a few cents rather than a whole jar. Also, packets of spices, you can grab a pinch from a packet of taco seasoning, lemon pepper chicken seasoning, italian dressing etc. and stretch them for multiple meals.
Herbs - are you or any neighbors growing parsley, chives, basil, etc? I always have a surplus this time of year


Chicken broccoli casserole - Your chicken + rice or small cubes of potato + frozen broccoli (or cauliflower or green beans - whatever is on sale, $1-2) + a can of condensed cream of something soup ($.79-99ish cents, store brand)
Fried rice - rice, scrambled egg, frozen mixed veggies (if you have some soy sauce and cooking oil already)
Ramen - you can throw in a little of your chicken or pork, some frozen veggies, a few slices of onion or garlic, or an egg and get a decently filling meal.
Baked potato + black beans and salsa - lots of fiber, will keep you full
Apple + peanut butter is a pretty hearty snack

Budget bytes blog has a lot of good ideas, especially if you have some condiments and spices already on hand

(your prices may be different depending on the store and your location)

If funds are really tight, I'd encourage you to look into food banks and food charities. You may get some staples (like peanut butter, bread, pasta ) for little to no money. If you remain unemployed, also look into food stamps.