For the roasted onions: Preheat the oven to 425 degrees F. Place the Vidalia and red onions on a foil-lined baking sheet and spray with canola oil spray. Sprinkle with salt and black pepper and roast for 20 to 25 minutes, giving a toss halfway through cooking.
For the vinaigrette: Meanwhile, pour the olive oil into a large bowl. Whisk in the orange juice, chili powder and lime juice and zest and season with salt and black pepper.
For the salad: Let the onions cool until lukewarm, about 10 minutes, then toss in the vinaigrette to combine. Add the spinach, sunflower seeds and avocado and gently toss together. Garnish with sundried tomatoes and anchovies, if desired. Serve immediately.