Preheat oven to 450 degrees. Line a baking sheet with aluminum foil; set aside. In a food processor, combine bread, parsley, and oil; season with salt and pepper. Pulse until coarse crumbs form.
Place salmon on prepared sheet; season with salt and pepper. Spread top of fillets with Dijon; top with crumb mixture, pressing gently to adhere. Roast until salmon is opaque throughout, 11 to 13 minutes.
Meanwhile, in a large bowl, combine lemon juice and remaining oil; season with salt and pepper. Add spinach and onion; toss to combine. Serve salmon with spinach salad.