How to Make It
Make the cakes Preheat the oven to 350°. Grease four 5-by-3-inch loaf pans with baking spray and line the bottom of each pan with parchment paper.
In a medium bowl, whisk the flour with the baking powder and salt. Using a Microplane, finely grate the zest of the 3 lemons; you should have about 2 tablespoons. Using a sharp knife, cut the skin and white pith from the lemons. Working over a fine-mesh strainer set over a bowl, cut between the membranes to release the lemon sections into the strainer. Discard the seeds and cut the sections into 1/2-inch pieces. Return the chopped sections to the strainer.
In a stand mixer fitted with the paddle, cream the butter with the sugar and lemon zest at medium speed until light and fluffy, about 4 minutes. Add the eggs one at a time, beating well after each addition and scraping down the bowl as needed. Beat in the vanilla. At low speed, beat in the dry ingredients and the crème fraîche in 2 alternating additions, scraping down the bowl as needed, just until combined. Using a rubber spatula, fold in the chopped lemon sections. Divide the batter among the 4 loaf pans and smooth the surface of each.
Bake for about 45 minutes, until a skewer inserted in the center of a cake comes out clean; shift the pans from front to back halfway through baking. Transfer to a rack and let cool completely, about 1 hour. Invert the loaves and peel off the parchment paper. Transfer to a platter.
Meanwhile, make the rhubarb In a medium saucepan, combine the sugar with the vanilla bean and seeds, orange zest and 2 cups of water. Bring to a simmer over moderate heat, whisking occasionally to help the sugar dissolve. Add the rhubarb and poach until just tender, about 6 minutes. Transfer to a bowl and let cool, about 30 minutes. Discard the vanilla bean and orange zest.
Serve the cakes with the poached rhubarb and syrup.
The lemon loaves can also be baked in one 9-by-5-inch loaf pan. Bake at 350 degrees for 55 minutes, tenting the loaf with foil after the first 30 minutes to prevent the top from getting too dark.