Step 1

Heat olive oil in a 12-inch cast-iron skillet over medium-high. Add onions, and cook, stirring often, until onions are slightly softened, about 4 minutes. Add squash and zucchini, and cook stirring often, until vegetables are lightly browned and softened, about 10 minutes. Stir in garlic, and cook 30 seconds. Add tomatoes, crushing with hands as you add to skillet. Stir in tomato puree, water, salt, pepper, and pasta, being sure to cover pasta with liquid. Let come to a boil. Reduce heat to medium, and cover. Cook until pasta is al dente, 16 to 18 minutes.

Step 2

Stir in arugula and 2 tablespoons of the basil, and cook until arugula wilts, about 1 minute. Top evenly with mozzarella pearls; cover and cook until cheese is melted, 1 to 2 minutes. Sprinkle with remaining 2 tablespoons basil, and serve.