Preheat the oven to 425°F (220°C) and line a baking sheet or lightly oil it.
In a large bowl, toss the sliced chicken with the butter or oil, chopped garlic, 3–4 tablespoons taco seasoning, and a pinch of salt and black pepper until evenly coated.
Add the sliced yellow onion, bell peppers, and poblano to the bowl; season with additional salt and pepper and toss to combine.
Spread the chicken and vegetables in an even layer on the prepared baking sheet.
Bake for 20–25 minutes, until the chicken is cooked through and the peppers begin to char, stirring once halfway through for even cooking.
While the sheet pan cooks, make the verde sauce: in a bowl combine 1/2 cup Greek yogurt, 1/4 cup mayo (or extra yogurt), 3/4 cup salsa verde, 1–3 tablespoons chopped pickled jalapeños, and 1/2 cup chopped cilantro; season with salt to taste.
Warm the tortillas. Fill each tortilla with the roasted chicken, peppers, and onions, then top with avocado, a spoonful of the verde sauce, more cilantro, and crumbled cotija or feta.
Serve immediately with lime wedges if desired.