Preheat oven to 450F. Move the rack to the middle.
Add the Greek yogurt, chilis in adobo sauce, lime juice, garlic, and salt to a blender or food processor. Blend until smooth. Pour dressing into a jar and set aside until needed.
Cut the broccoli into bite-size florets. Cut the mushrooms into halves or quarters, depending on size of mushrooms. Add the broccoli and mushrooms to a baking sheet and toss with 2 tablespoons of the olive oil, the garlic powder, and salt & pepper. Roast for 15-20 minutes.
Meanwhile, cut the cauliflower into bite-size florets. Add the cauliflower to a food processor (you may need to do more than one batch), and process on low speed until the cauliflower resembles rice or couscous.
Add the cauliflower rice to a skillet with a tablespoon of olive oil and some salt to taste. Cook for 10 minutes (or until it's as tender as you like) on medium heat, covered, stirring occasionally.
Slice the avocado and scallions. Assemble the bowls and serve immediately.