Heat the butter in a large skillet over medium heat. Add the onion and garlic, and cook 5 minutes, or until the onion is slightly tender. Stir in the zucchini, squash and saute just until it begins to soften, about 5 minutes. Mix in the tomatoes and raise the heat to medium-high. Cook, stirring often, until the zucchini and squash are tender but not mushy and the juices have evaporated. Sprinkle with Queso Fresco crumbles over zucchini/squash at a low simmer. Cover and cook the cheese until just tender, about a few minutes. Sprinkle with cilantro to serve.