Melt the butter in a large skillet over medium heat; cook and stir the onion in the melted butter until translucent, about 5 minutes. Add the garlic; cook and stir 1 minute more. Stir the tomatoes and 1/4 cup water into the mixture; cover. Cook until the tomatoes have reduced slightly, stirring occasionally, about 5 minutes.
Pour another 1/4 cup water into the skillet, and add the bouillon cube; stir until the bouillon is dissolved. Add the zucchini; continue cooking until the zucchini is tender, 5 to 7 minutes. Season with black pepper to serve.
