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Preheat oven to 375°F. Bring a large pot of water to a boil and fill a bowl with ice water. Add leeks to boiling water and cook for 3 minutes. Shock the leeks by transferring to the ice-water bath. Drain and slice leeks in half lengthwise.
Heat oil in an oven-safe cast-iron skillet or cocotte over medium heat. Add mushrooms and cook 3 to 5 minutes, stirring occasionally, until golden and slightly tender. Season with 1/4 tsp. salt and transfer mushrooms to a bowl.
Add butter and garlic to pan; cook 1 minute. Sprinkle in flour and whisk continuously for 30 seconds. Gradually stream in milk, whisking constantly, until combined. Increase heat to bring mixture to a simmer; cook 2 to 3 minutes, until slightly thickened. Add mushrooms back to skillet, along with thyme, remaining 3/4 tsp. salt, black pepper, and 1 tsp. lemon juice. Lay sliced leeks across the top of gratin, gently pressing to slightly submerge in milk mixture. Sprinkle breadcrumbs and Parmesan cheese overtop. Transfer to oven and bake for 35 to 40 minutes, until golden and bubbly.
Squeeze remaining 1 tsp. lemon juice overtop and garnish with additional fresh thyme before serving.