Bring a large pot of water to a boil. Add 8 ounces spaghetti; cook according to package instructions. Reserve ½ cup pasta water; drain the pasta.
Meanwhile, heat 2 tablespoons oil in a large skillet over medium-high heat. Add quartered mushrooms; cook, stirring occasionally, until golden brown, about 8 minutes.
Reduce heat to medium-low; add minced garlic and cook until fragrant, about 1 minute.
Stir in thawed spinach; cook, stirring often, until spinach is warm, about 3 minutes.
Add the pasta, the reserved pasta water, ¼ cup ricotta, 1¼ teaspoons salt and ½ teaspoon pepper; stir until combined and creamy.
Portion into 4 bowls and top with the remaining ¾ cup ricotta.
