In a bowl, combine 1/4 cup granulated sugar, the rhubarb, lemon peel, and 1 tablespoon lemon juice.
In a 10-inch frying pan over medium heat, melt 2 tablespoons butter; add brown sugar, bourbon, and remaining 1 tablespoon lemon juice; stir often until caramel has thickened and is medium brown, 7 to 10 minutes. Spread in a buttered 9-inch round cake pan (with 2-in.-tall sides).
Arrange pecans over caramel. Scatter rhubarb mixture on top.
In a bowl, with a mixer on medium speed, beat remaining 1/2 cup butter and 3/4 cup granulated sugar until blended. Beat in eggs until smooth. Beat in baking powder, vanilla, and salt, half the crème fraîche, then half the flour; repeat with remaining crème fraîche and flour. Drop spoonfuls of batter over rhubarb; spread smooth.
Bake in a 350° oven until cake is golden on top and a toothpick inserted in center comes out clean, 40 to 50 minutes. Let cool on a rack 15 minutes.
Run a knife between cake and pan sides. Invert onto a plate. Serve warm.