Wash hands with soap and water for 20 seconds.
Scrub the onion with a clean vegetable brush under cold running water. Finely chop the onion; set aside.
If not cleaned, wash the spinach under cold running water.
Rinse and clean the oysters.
Shuck oysters, reserving bottom shell; set aside.
Wash the counter and utensils that touched the raw shellfish. Wash hands with soap and water after handling raw shellfish.
In a large skillet, saute the onion in butter until tender. Add spinach; cook and stir until wilted. Remove from the heat; stir in the cheese, lemon juice, and pepper.
Spread kosher salt into two ungreased 15x10x1-in. baking pans. Lightly press the oyster shells down into the salt. Place one oyster in each shell; top each with 2 1/2 teaspoons spinach mixture.
Bake, uncovered, at 450°F until oysters are plump, 10-15 minutes. Cook until internal temperature reaches 145°F on a food thermometer.
Serve immediately.