Heat a portion of the oil in a large nonstick skillet over high heat until it just begins to smoke. Add the bell pepper and cook until the edges are lightly browned, about 2 minutes. Stir in the asparagus, salt, and pepper and cook until the asparagus is browned and almost tender, about 2 minutes. Stir in the shallot and cook until softened and the asparagus is crisp-tender, about 1 minute. Transfer the mixture to a large plate and let cool for 5 minutes—this rest prevents the hot vegetables from wilting the spinach too aggressively.

Whisk the vinegar, garlic, salt, and pepper together in a bowl. Whisking constantly, slowly drizzle in the remaining oil until the dressing is well combined and slightly thickened.

Toss the spinach with a small amount of the dressing in a bowl and divide among plates. Toss the asparagus mixture with the remaining dressing and arrange over the spinach. Top with the goat cheese and serve while the vegetables are still warm.