1 cup plain sour cream
1 tablespoon freshly squeezed lime juice
1 teaspoon lime zest
1/2 teaspoon kosher salt, plus more for seasoning
1/2 teaspoon freshly ground black pepper, plus more for seasoning
1 medium red onion, cut into 8 wedges
3 large portobello mushrooms, cut into 1-inch slices
Vegetable oil, for brushing the vegetables