Preheat oven to 400 degrees. Line a baking sheet with a large piece of foil and grease lightly.
Place fillets in a single layer on prepared baking sheet. Squeeze the lemon half to drizzle juice over the salmon fillets. Season both sides with salt and pepper to taste . Wrap the edges of the foil over the salmon fillets - this helps to keep them moist! Bake for about 15-10 minutes or until flakey and tender.
While salmon is baking, prepare the sauce. Add avocado, cilantro, lime juice, garlic powder, onion powder, and salt to a blender or food processor and pulse several times. Add milk or water 2-3 tablespoons at a time until mixture reaches a pourable consistency. Chill until ready to serve.
When salmon is fully cooked, drizzle with avocado sauce. If desired, serve salmon with additional chopped cilantro, fresh limes or lemons for squeezing, and rice.