1. Place a jelly-roll pan in oven. Preheat oven to 450° (leave pan in oven).
2. Place 1 1/2 teaspoons butter, oil, and crushed garlic in a small microwave-safe bowl. Microwave at MEDIUM (50% power) for 1 minute or until butter melts and garlic is fragrant. Cool slightly; discard garlic.
3. Carefully cut 2 (1-inch-thick) "steaks" vertically from center of each cauliflower head; reserve remaining cauliflower for another use. Carefully trim bottoms of maitakes, keeping the mushroom clumps intact. Carefully cut each maitake in half lengthwise. Brush tops of cauliflower steaks with oil mixture. Carefully place cauliflower steaks, oiled sides down, on preheated pan; brush tops of steaks with oil mixture. Bake at 450° for 14 minutes or until bottoms are browned. Remove pan from oven; carefully turn cauliflower steaks over. Gently brush remaining oil mixture on both sides of maitake steaks; place on pan with cauliflower, cut sides down. Bake at 450° for 13 minutes or until cauliflower is browned and done. Remove from oven; sprinkle evenly with 1/4 teaspoon salt and 1/4 teaspoon pepper.
4. While cauliflower bakes, bring milk and 1/2 cup water to a simmer. Gradually add polenta, stirring constantly with a whisk. Cook for 3 minutes or until thick, stirring constantly. Stir in remaining 1/2 teaspoon salt. Cover and keep warm.
5. Melt the remaining 2 tablespoons butter in a medium skillet over medium heat; cook 3 minutes or until browned and very fragrant. Add sliced garlic; cook 45 seconds, stirring frequently. Add wine; increase heat to medium high, and cook 1 minute or until about half of liquid evaporates. Combine broth and flour, stirring with a whisk. Add broth mixture to pan; bring to a boil. Cook 1 minute or until slightly thickened, stirring constantly; stir in capers, juice, thyme, and remaining 1/4 teaspoon pepper. Spoon about 3/4 cup polenta on each of 4 plates. Top each serving with 1 cauliflower steak, 1 maitake steak, and about 2 tablespoons sauce. Garnish with thyme, if desired.