Melt butter in a small skillet over medium heat; add garlic, and sauté 1 minute. Stir in breadcrumbs and 1/4 tsp. salt, and cook, stirring occasionally, 3 to 4 minutes or until lightly toasted. Remove from heat.
Arrange Brussels sprouts in a steamer basket over boiling water. Cover and steam 8 minutes or until crisp-tender. Add cauliflower. Cover and steam 2 to 3 1/2 minutes or just until tender. Transfer Brussels sprouts and cauliflower to a serving dish; sprinkle with lemon juice, pepper, and remaining 1/4 tsp. salt, and toss to combine. Sprinkle with breadcrumb mixture; serve immediately.