In a medium skillet over medium heat, warm 2 Tbsp. olive oil and 1 Tbsp. butter. When butter is melted, add 2 thinly sliced yellow onions. Cook, stirring occasionally, until onions begin to brown, 10 minutes. Peel, core and cut 2 firm-ripe pears into cubes. Add pears and 1/2 tsp. dried rosemary to onion mixture and continue cooking, stirring occasionally, until onions are browned and pears have softened, 10 to 15 minutes longer. Season to taste with salt and pepper.
Toast 8 slices of whole-wheat bread. Spread 4 slices with 1/3 lb. ripe Brie. Layer on 1 1/2 lb. sliced leftover Thanksgiving turkey, then pear-onion mixture and 3 oz. baby spinach. Top with remaining 4 slices of bread. Cut sandwiches in half and serve.