Preheat oven to 450°F. Line a baking sheet with foil and arrange polenta slices on it. Melt 4 tablespoons butter in a small saucepan. Brush both sides of polenta lightly with melted butter and roast until golden brown on edges, 25-30 minutes.
Meanwhile, make spinach. Melt remaining 2 tablespoons butter in a large skillet over medium heat. Add garlic and cook until fragrant, about 1 minute. Sprinkle in flour and cook, whisking, for 2-3 minutes. Whisk in cream and milk and bring to a gentle simmer, whisking, until thickened, about 5 minutes. Reduce heat to low and stir in spinach and parmesan. Season with salt and pepper. Keep warm.
Arrange polenta bites on a platter. Top with a spoonful of creamed spinach and a sprinkle of chopped bacon. Serve.