Combine the porcini mushrooms and boiling water in a bowl; cover and let stand 30 minutes. Drain porcinis in a colander over a bowl, reserving liquid. Rinse and chop porcinis.
Heat oil in a large skillet over medium-high heat until hot. Add onion; sauté 5 minutes or until lightly browned. Stir in tomato paste, and saute 30 seconds. Add porcini, button, and shiitake mushrooms, stirring to coat. Sauté 5 minutes or until mushrooms release moisture and darken, stirring once. Sprinkle mushroom mixture with flour; cook 1 minute, stirring constantly. Stir in reserved mushroom liquid; sprinkle with salt and pepper. Bring to a boil; cook 1 minute. Stir in parsley and minced garlic. Remove from heat.
Rub garlic halves on one side of each toast slice; sprinkle each slice with 2 tablespoons cheese. Top each with 3/4 cup mushroom mixture.