Slice 2 medium zucchini and halve or quarter 6 oz. mushrooms depending on size. Cut 1 yellow onion into quarters, then halve them. Dice 1 ½ bell peppers into chunks.
Preheat a large grill pan (or large skillet), over medium heat. Add cut vegetables to a mixing bowl. Add 2 tbsp. avocado oil and ½ tsp. each pepper, garlic powder, dried basil and kosher salt.
Toss vegetables and transfer to the pan. Cook for 3 to 5 min per side, flip and cook another 3 to 4 min until tender. Cook in two batches, then transfer to a plate.
While vegetables are cooking, slice 4 servings baguette. Whisk the Dressing together in a bowl (1 tbsp. olive oil, ¼ tsp. each kosher salt, pepper garlic powder, juice of ½ lemon, 1 tbsp. white wine vinegar, 1 tsp. dijon mustard, and ½ tbsp. each fresh parsley and fresh dill).
Lay out baguette slices in grill pan. Optionally brush with a bit of avocado oil and cook for 2 to 3 min per side until lightly golden in spots. Dice or slice 2 avocados.
To serve, plate the grilled vegetables along with torn baguette slices. Top with avocado and 1/3 cup castelvetrano olives. Drizzle with dressing. Serve with remaining ½ lemon cut into wedges.