In a small skillet, heat the oil over medium-high heat. Add the garlic and stir, lightly frying for about a minute, removing before the garlic gets too brown (it can be golden). Pour the garlic and oil into a mixing bowl and allow to cool slightly.
Add the red and yellow tomatoes, balsamic and basil to the bowl. Sprinkle with salt and pepper. Toss to combine, and then taste and add more basil if needed, and more salt if needed (don't over salt, though!) Cover and refrigerate for an hour or two if you have the time.
Cut the baguette into diagonal slices to allow for the most surface area possible. Melt 4 tablespoons of the butter in a large skillet and grill half the bread on both sides, making sure they're nice and buttery. Cook until golden brown on both sides. Repeat with the remaining 4 tablespoons butter and the other half of the bread.
To serve, give the tomato mixture a final stir, and then spoon generously over the slices of bread. Serve on a big platter as a first course or appetizer.