Preheat your oven to 400°F and line a large rimmed baking sheet with parchment paper for easy cleanup.
Prepare the quinoa by combining 0.25 cup dry quinoa with 0.5 cup water in a small saucepan; bring to a boil, then simmer covered for 15 minutes until fluffy.
Trim the tough, woody ends off the asparagus spears and arrange them on one half of the prepared baking sheet.
Pat the tilapia fillets dry with a paper towel to ensure a better texture and place them on the other half of the baking sheet.
In a small glass bowl, whisk together the olive oil, lemon juice, minced garlic, dried oregano, sea salt, and black pepper.
Drizzle the herb mixture over the tilapia and asparagus, using a brush or your hands to ensure everything is evenly coated.
Top the tilapia fillets with the fresh lemon slices to infuse the fish with bright citrus notes while it cooks.
Bake in the center rack for 12 to 15 minutes, or until the tilapia is opaque and flakes easily with a fork.
Divide the cooked quinoa between plates and top with the roasted asparagus and lemon-herb tilapia fillets.