Step 1Heat the oven to 375 degrees. Butter a large gratin dish. Sprinkle the garlic over the bottom.
Step 2Slice the potatoes about one-fourth-inch thick. Arrange half the slices, slightly overlapping, across the bottom of the pan. Strew the dandelion greens over the potato slices. Season with one-fourth teaspoon salt and a pinch of pepper. Arrange the remaining potato slices over the top. Season again with one-fourth teaspoon salt and a pinch of pepper.
Step 3In a small bowl, whisk the creme fraiche with the milk and pour the mixture evenly over the potatoes. Cover with the cheese.
Step 4Bake until the potatoes are easily pierced with a knife, the cream is bubbly and the cheese is browned, about 45 minutes. Serve immediately.