To make the crème fraîche, mix the cream and sour cream together in a bowl. Allow to sit at room temperature for 6 to 8 hours. Place a coffee filter in a funnel or line a fine sieve with paper towels, add the cream, and set aside to drain for 2 to 3 hours, until thick.
Mix in the lemon juice and store in the refrigerator for up to 1 week.
To make the pudding,