Combine all dried spices in a small bowl: chili powder, oregano, smoked paprika, cumin, salt, garlic powder, onion powder, black pepper, and cayenne pepper. Set aside. While preparing the spice blend, slice the green, red, and yellow bell peppers into 1/4-inch strips, cut the red onion into 1/4-inch thick wedges, slice the zucchini lengthwise then quarter each slice, and quarter the cremini mushroom caps. Arrange all cut vegetables on a large cutting board or platter for easy access during grilling.

Preheat your grill and grill topper to 350°F, allowing about 10-15 minutes for it to reach temperature. In a large bowl, toss the sliced bell peppers and red onion wedges with 1/2 tablespoon of avocado oil until evenly coated. This light coating helps prevent sticking and promotes even browning. I find that keeping the peppers and onions together for the first batch ensures they cook at the same rate and develop nice caramelization.

Once the grill reaches temperature, carefully spread the oiled peppers and onions in an even layer on the grill topper. Grill for about 15 minutes, stirring halfway through (around the 7-8 minute mark) to ensure even cooking and charring on multiple sides. The vegetables should be tender with some light charring at the edges when done.

While the peppers and onions finish cooking, toss the quartered zucchini with 1/2 tablespoon of avocado oil in a separate bowl. When the peppers and onions have been grilling for about 13 minutes, add the zucchini to the grill topper. After 2-3 minutes, add the mushroom quarters that have been tossed with the remaining 1/2 tablespoon of avocado oil. I like to add the mushrooms after the zucchini because they cook quickly and you want them tender but not mushy.

Once all vegetables are on the grill, sprinkle about 2 teaspoons of the spice blend from Step 1 evenly over everything and gently toss to coat. Continue grilling for another 2-3 minutes, allowing the spices to toast slightly and infuse into the vegetables. All vegetables should be tender with charred edges and caramelized spots—the total grilling time should be about 18-20 minutes from start to finish.

While the vegetables finish cooking, warm your corn tortillas on the grill or in a dry skillet over medium heat for about 30 seconds per side, or wrap them in foil and place them at the edge of the grill to stay warm. Transfer the finished grilled vegetables to a serving platter. Arrange the warm tortillas, lime wedges, fresh cilantro, and optional guacamole and sour cream on the table for your guests to build their own fajitas.