In a large skillet, heat 1 teaspoon oil over medium-high heat. Add rhubarb, onion, sugar and lemon juice; cook and stir until tender, 5-7 minutes. Stir in zest and lavender. Remove from the heat. Mash to a chunky consistency; set aside., Brush bread slices on both sides with remaining 4 teaspoons oil. Place bread on ungreased baking sheets. Broil 3-4 in. from heat until golden brown, 1-2 minutes on each side. Spread with goat cheese. Top with rhubarb mixture and pistachios.