The assumption has to be that one already has seeds. I maintain a nice collection of various seeds in my refrigerator, plus I'm already capable of producing my own seeds or starts for certain veggie crops -- beans, peas, radishes, lettuce, potatoes, sweet potatoes, taro, chives, dill, basil, cilantro, pipinola, tomatoes, peppers, eggplant, tomatillo, amaranth. So the very first thing I'd do is run to the Ace Hardware and buy more seeds of chard, beets, spinach, cabbage, cucumber, watermelon, pumpkin, squashes, broccoli, bok choy, onions, carrots, turnips, and rutabaga. Other people would be running to the supermarket to stock up on spam, rice, milk, and toilet paper, so I'd have little competition if I got a headstart. (Yes, I still need to get in the habit of producing my own seed for these other crops.) Plus I'd most likely pick up packets of some hybrids that I like, since I wouldn't be able to produce their seeds for myself. But importantly I'd buy bulk seed packets for sprouting.
The first thing I'd do is grow sprouts. Within a few days I'd have fresh, crunchy sprouts to add to sandwiches, stir fry, omelettes, etc. These could hold me over until other fresh crops came in and help extend my pantry of stored foods.
I'd also start a few trays of my sprouting seeds with the idea of harvesting micro greens in 10-20 days. This would carry me after the initial sprouts and until the first baby greens are ready.
The next crop to arrive, assuming I started them when I first started the sprouts and micro greens, would be young baby greens. Lettuces, beet & chard greens, assorted Asian greens, broccoli greens, radish greens, onion greens, etc. These could be harvested 20-30 days after starting, with repeated harvests for 3-4 weeks. Plus right about this time frame I'd be harvesting radishes. These young greens could carry me until the early crops are ready. And they could be used as salads and in all sorts of cooked meals.
The next crops would be those that are ready in the 50-60 day range. I could be making small first cuttings of a variety of young herbs, such as cilantro, dill, parsley, basil. Plus light harvests of beet greens, chard, kale, broccoli and cauliflower leaves, collards, cabbage, spinach, amaranth, tatsoi, bok choys. Baby bok choy types would be ready for harvesting. In another week or so the harvests could be more serious, plus greenbeans and peas would be available for their first pickings. I could gently harvest some potatoes from the early varieties. Plus I'd have the tender tips to harvest from the sweet potato and pipinola vines.
Within a couple more weeks I'd have a bonanza to choose from. Cucumbers. Young baby squashes, pumpkins, and edible gourds. Chinese broccoli. Plenty of leaves of bok choy and Chinese cabbage although they would not be heading up yet. Same for regular cabbages. Beets. Potatoes. Turnips. Daikon. Young peppers. Cherry tomatoes. Baby eggplant. Amaranth. The first tomatillos.
In another month I could be looking at young carrots, parsnips, and baby sweet potatoes. But since I'd be well supplied with other garden foods, I'd most likely leave these to grow to maturity for bulkier harvests. I'd also have plenty of green onions at this stage.
In actuality, I already have a producing garden. So I wouldn't need to grow sprouts, micro greens, and baby greens. But if I wasn't already growing my own food, this is the route I'd be going.
Currently I forage for some of my foods. I assume that this would be a no-go, since I'd be in competition with lots of people searching for food after their rice and spam began running out. But I'd be in a fine position for trading. Of course I'd have to watch out for garden raiders sneaking in the night. So I'd simply grow easy abundant crops out by the street for the raiders. Things like lots of greenbeans and radishes.
Hopefully I'll never see such a food crisis. But one never knows.