In large skillet, melt butter over medium-high heat. Add onions and garlic; sauté until onions are translucent (about 2-3 minutes). Add in spinach, thyme, salt and pepper; sauté until all liquid is evaporated.
Stir in Alfredo sauce and sherry; remove from heat. Spread small amount of Alfredo sauce across bottom of a 13x9-inch baking dish that has been sprayed with cooking spray.
Top with 1/3 noodles, ½ shrimp, ½ crab, 1/3 Alfredo sauce and 1/3 cheese. Repeat layers ending with 1/3 noodles, 1/3 Alfredo sauce and 1/3 cheese.
Bake, covered, in a preheated 375°F oven 45 minutes. Remove cover and continue baking until golden brown and bubbly (about 10-15 minutes). Allow to sit 10 minutes before serving.