Do this with a regular onion, especially if you've already got one in the pantry trying to sprout. As it grows you'll get onion greens that work just like scallions in any recipe. Let it go to seed, now you have infinite onions, but depending on your local climate and luck, leave your original onion bulb to winter, and shoot again, and it has probably split into new bulbs, so you'll probably get 2 new onions from the plant, plus onion greens, plus seeds. Eat one bulb, and leave the other bulb to grow more onion greens.

I've never bothered using the seeds, I just keep a bulb or two in the pot. Been 5 years. I still buy onions if I want something like onion jam or French onion soup, where I need like 1kg of onions. But Ive never had to buy scallions, and I've got onion flavour all year long through onion greens (you can dehydrate them, and freeze them really easily too, to store them when you have more than you can use)

I also highly recommend throwing peas into a large tray of soil. Litteraly just grab a bunch of aluminium foil disposable oven pans if you need to, stab some holes in them with a knife, an inch or two of soil, some dried whole peas or fresh garden peas, a sprinkle of more soil or just a wet sheet of kitchen roll/paper towel on top.

You probably won't get peas, but you'll have tons of pea tendrils for salads. On my balcony it's the only "salad green" I've had any luck growing. I have a pretty black thumb. I can't even manage to sprout chia seeds without them moulding, and I've never been able to grow mint despite broad casting mint seeds directly into my garden, urging the gardening gods to spite me with weedy mint but no dice.

When I buy peas, 4/5ths go in the fridge to eat, the other 5th gets planted, and I'll get ~10 dishes from the tendrils vs 1 dish from the peas. Nutritionally the peas have more protein, but lentils are cheap, salad is expensive, so this works for my budget.