Gather the ingredients.

Melt butter in a large skillet over medium heat. Add onion; cook and stir until onion is tender, about 5 minutes.

Add garlic; cook and stir until fragrant, about 1 minute more.

Stir tomatoes and 1/4 cup water into the mixture; cover. Cook, stirring occasionally, until tomatoes have reduced slightly, about 5 minutes.

Pour another 1/4 cup water into the skillet. Add bouillon cube and stir until bouillon is dissolved. Add zucchini; continue cooking until zucchini is tender, 5 to 7 minutes.

Season with black pepper and serve.