Preheat oven to 400 F. Line a rimmed small baking sheet (I used half sheet) with parchment paper. Place rainbow trout skin side down in the center of the baking sheet. Brush half of olive oil amount over the top of trout.
Toss the asparagus (ends trimmed) with the remaining half of olive oil. Place asparagus all around trout on a sheet pan.
Sprinkle the trout and the asparagus with salt and pepper. Add minced garlic on top of the trout fillet and asparagus. Top with lemon slices.
Bake for about 15 minutes at 400 F in the preheated oven until the trout is cooked through. Remove from the oven. Remove lemon slices from the trout before serving.