Make the Crema Fresca ahead of time: mix cream and sour cream together. Cover and let stand at room temp. until thickened, 8 hours or overnight. Chill until ready to use. Char the chili peppers over a gas flame until blackened on all sides. Wrap them in a plastic bag and let stand for 10 minutes to steam. Peel and core the chilies. Remove seeds, rinse and pat dry. Preheat oven to 180ºC . Generously butter a 23 cm pie pan (preferably porcelain or stoneware with 5 cm sides). Open up chili peppers and arrange around sides of pan skin side down, point toward center of pan, extending about 1 cm above rim. Cover bottom of pan with chilies. Finely grate cheese with onion and garlic in a processor using on/off pulses, about 30 seconds. Add eggs and salt. Process until smooth, stopping to scrape down the sides, about 15 seconds. Mix in the Crema Fresca (the Mexican equivalent of creme fraiche). Pour this filling over the chilies. Curl the edges of the chilies over the filling. Bake until golden brown and a knife inserted in the center comes out clean, 45 to 50 minutes. Cover chili edges with foil to prevent burning, if necessary. Cool for 5 minutes before cutting. Serve hot or at room temperature. Serves 6 main course servings.