Bring the cream, half-and-half, lemon zest, orange zest, and cinnamon to a simmer in a medium saucepan over medium heat. While the mixture heats, whisk the eggs and 1/4c up sugar in a large bowl. Slowly add the hot liquid to the egg mixture in a steady stream, whisking constantly.
Lay the brioche slices in a single layer in glass or ceramic baking dishes. Slowly and evenly pour the egg-cream mixture over the bread, scattering the zest and cinnamon evenly over the bread. Cover tightly with plastic wrap and refrigerate overnight.
Uncover the soaked brioche and discard the zest and cinnamon. Heat a thin sheen of clarified butter in a large nonstick skillet over medium heat. Add a single layer of soaked brioche slices and cook, turning once, until evenly browned on both sides and cooked, about 6 minutes. Transfer to a half-sheet pan. Repeat with the remaining brioche slices, coating the skillet with clarified butter between batches. If the brioche browns before the center is heated through, reduce the heat.
Spring a teaspoon of sugar evenly on top of each cooked slice. Using a kitchen blowtorch, caramelize the sugar on each piece of brioche as you would crème brulée. Serve immediately.