2large or 4 small portobello mushroom caps, stems trimmed and gills removed
1small eggplant or 1 baby eggplant, sliced crosswise into 1/2 inch slices
1large or 2 small red bell peppers, cut into 2-inch strips
1large or 2 small red onions, sliced crosswise into 1/2-inch slices
4burger buns, toasted
To make the marinade, whisk all marinade ingredients together.
Marinate the vegetables for at least 4 hours and up to overnight. I recommend overnight for extra flavor. You can spread the vegetables in a casserole dish or two and pour the marinade over them, flipping them to ensure that both sides of the veggies get coated. Or—my preference—marinate them in gallon sized silicone or plastic storage bags (this is especially easy if you're throwing them in the fridge overnight).
Heat a grill pan over medium high heat. Remove the veggies from the marinade, reserving the marinade. Grill them for 6-10 minutes, flipping now and then so that both sides grill evenly. You can baste the vegetables with the reserved marinade every so often as you flip them. When all of the vegetables are soft and nicely grilled, remove them from heat. You may need to remove ones that cook more quickly (eggplant) before others (peppers).
Alternatively, you can roast the vegetables on a foil or parchment lined baking sheet at 425F for about 25-35 minutes, flipping and basting them halfway through, or until they're tender and browning.
To serve, pile the vegetables on burger buns in hearty stacks. Feel free to add any condiments, herbs, greens, or garnishes you like!
Store the grilled vegetables in an airtight container in the fridge for up to 4 days.