Place an oven rack in the second highest position (about six inches from broiler). Preheat oven to broil setting. Meanwhile, thaw shrimp and scallops, if necessary, and pat dry. Season with sea salt and black pepper.
Add butter to a ½ sheet pan and place pan on positioned oven rack until butter melts (about one minute).
Once butter is melted, remove pan from oven and add olive oil and lemon juice then carefully stir in garlic, onion, and crushed red pepper. Add tomatoes, shrimp and scallops to pan and stir to coat in butter mixture. Place pan back in oven for three minutes.
Stir contents of pan to ensure no sticking and even cooking. Return to oven for two more minutes then carefully remove.
Stir spinach into pan, ensuring it gets coated with butter mixture so that it wilts.