Cook pasta according to package directions, reserving 1 cup pasta water before draining.
While pasta cooks, halve cherry tomatoes and mince garlic, then sauté garlic in oil over medium heat for 1 minute, add tomatoes, and cook 5 minutes until softened.
Drain tuna and stir into tomatoes with spinach, cooking 2 minutes until spinach wilts, then add pasta and toss, adding pasta water to loosen sauce as needed.
Season with salt, pepper, and red pepper flakes, then serve warm.
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