1. Preheat oven to 350°F. Coat 12- x 8-inch oval baking dish with cooking spray. Place 1 bag spinach in microwave-safe bowl, cover, and microwave on high power 2 minutes, or until spinach is wilted. Transfer to kitchen towel, and squeeze out as much water as possible. Repeat with remaining bag of spinach. Coarsely chop, transfer to bowl, and season with salt and pepper, if desired.
2. Steam Brussels sprouts in steamer 6 to 8 minutes, or until tender. Transfer to shallow bowl, and cool.
3. Melt 1 Tbs. butter in skillet over medium-low heat. Add onion and garlic, and sauté 3 minutes. Add to spinach in bowl (do not stir).
4. Melt 1 1/2 Tbs. butter in same skillet over medium-low heat. Whisk in flour, and cook 1 minute. Gradually whisk in milk. Increase heat to medium, and cook 2 minutes, or until sauce thickens and boils, whisking often. Whisk in 2 1/2 tsp. horseradish, and season with salt and pepper, if desired.
5. Mix 1/4 cup sauce and 1 1/2 tsp. Dijon mustard into Brussels sprouts in bowl. Stir remaining sauce into spinach mixture.
6. Grind bread cubes in food processor until reduced to rice-size crumbs. Wipe out skillet, then melt remaining 1 Tbs. butter in skillet over medium heat. Whisk in remaining 1 1/4 tsp. horseradish and 1 1/4 tsp. mustard. Add breadcrumbs, and sauté 5 to 6 minutes, or until coated with butter mixture and beginning to brown.
7. Spread spinach over bottom of prepared baking dish. Arrange Brussels sprout halves cut sides down in concentric circles over spinach. Top with breadcrumbs, leaving some Brussels sprouts visible. Bake 25 to 30 minutes, or until sauce is bubbling around edge. Let stand 5 to 10 minutes before serving.