For the garlic custard: Bring the cream to a boil in a medium saucepan over medium heat. Lower the heat and then simmer until liquid is reduced to 1 cup plus 2 tablespoons. Whisk in the roasted garlic. In a mixing bowl, whisk the egg yolks lightly while drizzling in the cream mixture, and season with salt and pepper. Cover and set aside to cool.
For the tamale tart: In a small saucepan, steam the red bell pepper over boiling water until soft, about 10 minutes. Drain and transfer to a blender or food processor; blend until smooth. You should have about 3/4 cup puree; if necessary, add a little water to make up the quantity.
In a medium bowl, combine the masa harina, cornmeal, cayenne, cumin and salt. In a large bowl, using an electric mixer at medium speed, whip the shortening until light and fluffy.
Gradually beat the dry ingredients into the shortening to form a smooth dough. Beat in the ancho chili puree and the red bell pepper puree. Form the dough into a disk and pat into a 9-inch tart pan with a removable bottom, pressing the dough evenly over the bottom and up the sides. Fill the tart pan with the reserved garlic custard and cover with plastic wrap.
Place a round metal cooling rack inside a wok and pour in the water; the rack should fit at least 3 inches above the water. Bring the water to a boil over high heat, then turn off the heat. Place the wrapped tart pan on the rack and reheat the water until it boils again. Lower the heat to simmer, cover the wok, and steam until the custard is set but still trembles slightly, 25 to 30 minutes. Using pot holders, carefully lift the tart pan off the rack. Remove the plastic wrap and the sides of the tart pan. Transfer to a serving platter.
For the crabmeat: In a large non-reactive skillet, heat the olive oil over high heat until lightly smoking. Add the onion and cook, stirring for 1 minute. Stir in the crabmeat, tomatoes, cilantro, serrano and lime juice, and cook until warmed through, about 2 minutes. Season with salt. Using a slotted spoon, cover the top of the tart with the crab mixture. Cut the tart into wedges and serve warm.