Preheat oven to 350°F. Butter a 9x13-inch ovenproof dish. Cook pasta in a large pot of boiling salted water until just al dente, about 8 minutes; drain. Run pasta under cold water for 1 minute; drain well.
In a wide pot, melt the butter over medium heat. Whisk in flour and cook 1 minute, whisking. Whisk in milk & garlic. Cook over medium-low heat, whisking occasionally, until mixture thickens, about 10 minutes. Season with salt & pepper. Add cheeses, thyme & tomatoes. Stir until cheese has melted. Stir in reserved pasta.
Transfer to buttered dish. Sprinkle with breadcrumbs. Bake until top is golden brown, 20 to 30 minutes.