Preheat the oven to 450 degrees F (230 degrees C). Line a baking sheet with parchment paper.
Place potatoes on the baking sheet and toss with olive oil, salt, and pepper.
Roast in the preheated oven until starting to soften, about 10 minutes. Add onion and peppers. Continue roasting until vegetables are crispy, about 25 minutes.
While vegetables are roasting, make dressing. Combine tomatillos, mayonnaise, cilantro, lime juice, chile peppers, garlic, and chile powder in a small bowl.
Toss roasted vegetables with cotija cheese, then toss with dressing. Serve immediately.
