Cook pasta according to package directions, omitting salt and fat. Drain in a colander over a bowl, reserving 1/4 cup pasta water.
Heat a large skillet over medium-high heat. Add oil, onion, red pepper, and garlic; sauté 2 minutes. Add pasta, reserved 1/4 cup pasta water, ricotta, lemon juice, salt, and black pepper; cook 1 to 2 minutes. Remove from heat.
Combine spinach, arugula, and pasta mixture in a large bowl, stirring until spinach begins to wilt. Stir in watercress. Top with pecorino cheese.
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