Cook pasta according to package directions, omitting salt and fat; drain.
Heat oil in a large oven-safe skillet over medium-high. Add mushrooms; cook 4 minutes or until lightly browned. Stir in 1 tablespoon thyme and salt; cook 2 minutes. Sprinkle flour over pan; cook 1 minute. Add milk; bring to a simmer. Add pasta, Gruyère cheese, and 1 ounce mozzarella cheese, stirring until cheese melts. Stir in spinach. Spread pasta mixture in an even layer in pan; sprinkle with remaining 1 ounce mozzarella cheese and panko.
4. Bake at 375°F for 10 minutes. Carefully remove pan from oven; sprinkle with remaining 2 teaspoons thyme.
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