Heat the oven to 400°F. Put half the measured fat on one small uncoated stainless-steel sheet pan, or on one side of a large pan. Warm the empty pan only until the fat melts.
Halve small Brussels sprouts and quarter large ones. Dice the yellow onions.
Toss the onions in the first portion of melted fat with the measured salt for the onions and spread them into one layer.
Roast the onions for 25 to 30 minutes before adding the Brussels-sprout pan. This is their head start, not their full cook time.
Melt the remaining fat on a second small sheet pan, or on the other side of the large pan. Toss the Brussels sprouts with the fat and their measured salt, then arrange every piece cut-side down in one layer.
Roast the Brussels sprouts for 25 to 30 minutes. During this second roast, toss the onions about twice so darker pieces do not burn while lighter pieces brown.
Remove each pan when its vegetables are deeply browned without burned edges. If one finishes first, transfer that vegetable to a covered pan on the stove over the lowest heat.
Fill a serving bowl with the roasted Brussels sprouts and top them with the deeply browned onions. Serve hot.