In 10-inch nonstick ovenproof skillet, heat oil over medium heat; fry squash, onion and garlic until squash is tender-crisp, about 5 minutes. Stir in spinach.

Meanwhile, in bowl, whisk together eggs, 1 tbsp water, salt and pepper; pour over vegetables in skillet. Sprinkle with Monterey Jack cheese; cook over medium-low heat until bottom and side are firm but top is still slightly runny, about 7 minutes.

Place skillet under broiler; broil until frittata is golden and set, about 2 minutes.

Run rubber spatula around edge of frittata to loosen; slide onto large plate and cut into wedges.    

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Variation:  Add 1 Cup cubed smoked sausage, smoked turkey or smoked chicken with the spinach