Preheat oven to 400°F. Heat oil in a medium saucepan over low; add onion, and sauté until transparent, 4 to 5 minutes.
Combine onion, lemon zest, crabmeat, crème fraîche, cream, thyme, red pepper, salt, and black pepper. Pour mixture into a gratin dish, and top with cheese. Bake until top is brown and bubbling, 30 to 40 minutes. Serve with crackers or toast.
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