Adjust oven rack to middle position and preheat oven to 400°F. Melt 2 tablespoons butter in a large saucepot or Dutch oven over medium-high heat. Add fennel and onion, season with salt and pepper, and cook, stirring occasionally, until vegetables are softened and translucent, about 10 minutes, reducing heat if it starts to brown. Add garlic and cook, stirring constantly until fragrant, about 30 seconds.