For those of us not raised on a farm (which is the vast majority of Americans), it can be daunting to think of where to start when remaking our homes into the source of our food. You may have had a few vegetable growing experiences, the taste of a tomato fresh from the garden, or an apple or an orange right off the tree, but a few fruits here and there don't add up to much of a meal. To get some real nutritional value out of your plot of land, you are going to have to have something growing that you can count on day in and day out.
Which is why I am going to start in an unusual place -- those ingredients that keep cropping up over and over, like onions and peppers and herbs like parsley and thyme. Maybe add carrots and celery to the list as well. This combination of a few vegetables and herbs is the starting point in many cuisines. In French cuisine, a mirepoix is some celery, carrot, and onion, chopped up and sauteed. It is a start for soups, stews, casseroles, you name it. If you have these growing in your garden, you can always at least get dinner started, then add whatever else you have on hand.
Onions are a must have for the food security garden -- specifically the Egyptian walking onion variety. Bulbing onions are too much work, you have to harvest them in the summer and then dry them and store them, why not just keep a stand of Egyptian walking onions going and clip them as needed? They are fairly winter hardy, so unless you are in the frozen northland, a bed of them could keep you going through the winter. The only time I have found them lacking is in early summer, when they are putting all their energy into making new topsets, most of which are too small to cook with.
Peppers are perennial plants, although most American gardeners think of them as an annual. When I lived in coastal California (where it never freezes), I had a hedge of hot peppers outside the kitchen door. Jalapenos, tabasco, serrano, and pequin at the ready any time they were needed. With a little planning and preparation, you can lift your pepper plants out of the garden with winter approaching and keep them going if you have a sunny indoor location.
Carrots and celery are biennials, so they can stay in the ground and be harvested as needed. I prefer the European celeriac to the American fondness for stalk celery; it is less fussy in the garden and you can clip the small stalks as needed while you are waiting for the root to grow to harvesting size. Carrots can go anywhere in the garden. They are good companions for every other plant, although there are some things that will cause them to come up stunted. Parsley, if you get the kind that makes a carrot-like root is another biennial that can wait in the ground until needed.
Think about the vegetables that you like to have on hand all the time. What's sitting in your freezer? A sack of broccoli and carrots that you put into stir-fries, casseroles, the crock-pot? How difficult would it be to have those vegetables growing in the garden, ready to harvest any time? For true food security, you have to quit thinking in terms of a big harvest and look at your garden space as one big pantry.