Turn broiler or grill to high. Brush salmon with olive oil and mustard, and season wit salt and pepper. Place in broiler pan or on grill with skin side down, and cook until flaky throughout, about 8-10 minutes.

Heat a small nonstick skillet on medium-high, drizzle about 1/2 tbsp of olive oil. Add asparagus, season with salt and pepper, and stir frequently until softened. Remove to a plate, arrange spinach alongside, drizzle with more oil and vinegar, then season with salt and pepper and serve with salmon.