1. Preheat oven to 450°. Zest lemon. Transfer the zest to a food processor. Cut lemon into 4 wedges; set aside. Sprinkle salmon with 1/4 teaspoon salt and 1/4 teaspoon pepper.
2. Coarsely chop 1 garlic clove and add it to the food processor. Thinly slice remaining garlic and set it aside.
3. Tear the bread into chunks and add to the food processor along with mint, 1/4 teaspoon salt, and 1/4 teaspoon pepper. Process until coarse crumbs form and transfer to a small bowl. Stir in 1 teaspoon oil and set aside.
4. Heat 2 teaspoons oil in ovenproof skillet over medium-high heat. Add salmon, skin-side up. Cook 3 minutes. Turn fish with spatula, pat crumbs over the surface, and transfer pan to the oven. Roast 5 minutes.
5. While salmon cooks, heat remaining tablespoon oil in a large skillet over medium-high heat. Add sliced garlic and red pepper flakes. Sauté 1 minute, until garlic is lightly browned. Add spinach. Stir just until wilted, season with remaining 1/4 teaspoon salt, and toss. Divide among 4 dishes. Divide salmon into 4 portions; place fish on spinach. Serve with lemon wedges.