Rinse the quinoa under cold water and combine with 0.5 cup water in a small saucepan; bring to a boil, then reduce heat to low, cover, and simmer for 15 minutes.
Preheat your grill or a cast-iron grill pan to medium-high heat.
Finely mince the garlic clove and roughly chop the fresh parsley.
In a small bowl, whisk together the olive oil, juice from the half lemon, minced garlic, dried oregano, sea salt, and black pepper to create a marinade.
Brush the salmon fillet and the trimmed asparagus spears generously with the prepared lemon-herb mixture.
Place the salmon on the grill skin-side down and cook for 4-5 minutes, then flip and cook for another 3-4 minutes until the center is opaque and flakes easily.
Grill the asparagus alongside the salmon for 3-5 minutes, turning occasionally until tender and lightly charred.
Fluff the cooked quinoa with a fork and serve the grilled salmon and asparagus over the top, garnishing with the fresh parsley.