Put the bread on a baking sheet and toast until crisp and the edges are golden, about 6 minutes. Brush the bread on one side with 1 tablespoon of the olive oil while still hot.
Heat the remaining 2 tablespoons olive oil in a large skillet over medium-high heat. When the oil is hot, add the Brussels sprouts and season with salt and pepper. Let sit for a minute, then toss. Continue to cook and toss occasionally until the sprouts are lightly browned all over, 5 to 6 minutes. Add 1/3 cup water, reduce the heat so the water is simmering and cover the skillet. Cook until the sprouts are tender, 5 to 6 minutes. Meanwhile, stir together the balsamic, honey, red pepper flakes and 1/2 teaspoon salt in a small bowl.
When the sprouts are tender, uncover and add the balsamic mixture. Simmer rapidly until the sauce reduces and glazes the sprouts, 1 to 2 minutes. Add the butter and sprinkle with the lemon zest. Toss to coat the sprouts in the melted butter; they should be coated in a glossy glaze at this point.
To serve, spread the ricotta on the toasts. Top with the sprouts. Garnish with the pistachios and parsley leaves.