This isn’t rocket science.
You have to marinate them.
And you have to start with some good caps. Shriveled caps don’t work for me – they’re tough. I don’t mean fully dried ones, I mean ones that are old enough to shrivel. Skip those and go for some good, fresh, fleshy caps. The shriveled ones work better chopped up.
So get some good caps (one per burger), clean (use a damp paper towel – mushrooms absorb water so running them under the tap will make them bloat), pat dry, and put into a ziplock bag (or a container, whatever suits your fancy – just make sure it seals) with your marinade of choice. Good options include garlic/red wine vinegar, balsamic vinegar, teriaki sauce, even Worcestershire sauce. Choose something that you like. If you need a recipe try this:
1/4 cup red wine vinegar (I use marsala in a pinch)
a tablespoon or two of crushed garlic
a few tablespoons of Worcestershire sauce (to taste)
a teaspoon of crushed red pepper (to taste, more if you really want some heat)
Mix up in the bag (or bowl – whatever), and then add the caps. Let them sit for 10 minutes on a side, then remove. If they marinate for too long they will get mushy.
Toss on the grill (start with the smooth side on top) and grill on each side for about 4 minutes. Put on a bun. If you’re making cheese burgers, top the portobello with cheese after your first flip and it’ll melt nicely (try Swiss!).
Serve with the regular lettuce, tomato, etc.