Preheat oven to 425°F. Line one baking sheet with foil or parchment for chicken/asparagus.
Pat chicken breasts dry. Rub with avocado oil, then season both sides with lemon juice, salt, pepper, garlic powder, smoked paprika, and oregano. Place on the baking sheet.
Bake chicken for 15 minutes. Turn oven off (do NOT open door) and let residual heat finish cooking for 5–10 more minutes until internal temperature reaches 165°F.
While chicken bakes, bring 3 quarts water to a boil in a medium pot. Prepare a large bowl with ice and cold water for blanching.
Add asparagus to boiling water; blanch 3 minutes until bright green and crisp-tender. Immediately transfer to ice bath for 30 seconds to stop cooking. Drain and pat dry.
Transfer blanched asparagus to the same (or second) baking sheet. Drizzle with avocado oil, season with salt and pepper. Bake at 425°F for 5 minutes until lightly charred and tender.
In the same boiling water, add fingerling potatoes. Boil 10 minutes until fork-tender but not mushy. Drain.
Make a foil basket on a baking sheet: Crisscross two large sheets of foil, create a shallow pouch. Add boiled potatoes, 4 tbsp butter pieces, smashed garlic cloves, rosemary sprigs, lemon wedge, salt, pepper, and a drizzle of avocado oil. Seal loosely but tightly enough to trap steam.
Place foil packet on a baking tray. Switch oven to low broil. Broil potatoes 15 minutes until edges crisp and buttery flavors infuse (shake halfway if needed).
Plate: Slice chicken if desired. Arrange chicken, roasted asparagus, and potatoes on plates. Drizzle lightly with warm lemon beurre blanc.
Enjoy! Its okay if you lick the plate, I wont tell anyone ;)