How do I get more onion flavor into my focaccia?

I ran into the same wall with mine, and the fix turned out to be cooking the onions down before they ever see the dough. Raw or lightly salted slices just don't have the intensity to survive a hot oven. A lot of the sharp, punchy flavor in raw onion is volatile stuff that bakes off, and what's left behind is mild and a bit sweet, which is why your loaf reads as 'nice bread with a hint of onion' no matter how much you pile on.

What worked for me was slow cooking sliced onions in a generous glug of olive oil til they were soft, golden, and just starting to caramelize at the edges. Maybe 30 to 40 minutes on medium low, with a pinch of salt early on. Then I spread them over the dough with all that oniony oil, pressing them into the dimples so they're half sunk in rather than sitting on top. Pressing them in matters, it keeps them from drying out and they toast in oil instead of charring.

And don't toss that oil you cooked them in. Brush the top of the dough with it before baking. Onion flavored olive oil carries flavor way better than the slices themselves do, and it keeps the exposed bits from turning bitter.

If you want to go further, some people fold softened onions into the dough during the last fold so the flavor runs all the way thru. I've tried it and it works, though I find it makes the crumb a little gummy where the onion sits, so I mostly stick with the top method plus the infused oil. One more thing, shallots or red onions cooked down the same way give a deeper flavor than yellow, at least in my oven. Worth an A B test if you've got both sitting around.

Another trick from my rotation: a spoonful of onion powder in the flour itself. I use about a teaspoon per 500g of flour in the base dough, so every crumb carries some allium even before the toppings land. It dissolves into the dough during the fold and survives the oven because it's allready dry, nothing volatile to bake off. Combined with the cooked-down slices on top it gets you that punch you're chasing. I've also had good luck scattering thinly sliced scallion greens over the top five minutes before the bake ends, they keep some bite and a fresh edge the sweet caramelized ones can't give you.

What finally did it for mine was blitzing the cooked onions with the oil into a rough paste and spreading that on the dough before the slices go on. Gets the flavor everywhere instead of just where the pieces sit. I still scatter some sliced ones on top for looks. Also, chopped scallions thrown on right when it comes out of the oven give a sharp fresh bite the bake can't kill, that combo is my go to now.

One thing I stumbled into by accident was baking the loaf bare for the first stretch, then adding the onions partway through, maybe ten or so minutes in once the surface has set. Less time in the oven means less of the flavor drifting off, and the slices still brown fine on a hot crust. I also save my onion skins and root ends, simmer them in the oil on the lowest heat for twenty minutes, then strain. The oil comes out amber and surprisingly sweet, and brushing that on before and after the bake does more for the overall taste than anything else I have tried.

Update: sorted it out. Went with the cooked-down onion paste under the slices like a couple of you suggested, and that plus the onion powder in the dough finally gave me the punch I was after. The raw slices were losing most of thier bite in the oven, so this was the missing piece.