Salmon Burger with Baby Spinach:  2 hamburger buns   1 can (7.5 ounces) salmon, drained and flaked   1/2 celery stalk, finely chopped (2 tablespoons)   1/2 shallot, finely chopped (1 tablespoon)   1 tablespoon mayonnaise, plus more for serving   Coarse salt and ground pepper   2 teaspoons extra-virgin olive oil   Baby spinach, for serving
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