Halving some of the tomatoes gives this side a range of textures.
In a large saucepan, heat butter over medium. Add celery, onion, and 1/2 cup water; season with salt and pepper. Cook, stirring occasionally, until water has evaporated and celery is tender, 7 to 10 minutes.
Add tomatoes; cook until tomatoes are soft and juicy, 3 to 5 minutes. Stir in basil; season with salt and pepper.