Heat a grill pan over medium-high heat. Combine 1 1/2 tablespoons butter and 1 tablespoon sage in a small bowl, and spread on both sides of bread. Sprinkle with salt, and grill until golden and crisp, about 30 seconds per side.
Melt 1/2 tablespoon butter with 1 tablespoon oil in a large skillet over medium-high heat. Add shallots, and cook, stirring, until soft, about 2 minutes. Add 1/2 of the mushrooms, season with salt and pepper, and cook, stirring occasionally, until soft, about 5 minutes. Transfer to a bowl. Repeat with remaining butter, oil, and mushrooms. Return reserved mushroom-shallot mixture to skillet with remaining sage, and cook for 1 minute. Stir in vinegar, and cook until it evaporates. Serve warm over the grilled bread.