1. Prepare sauce: Melt unsalted butter in a medium saucepan over medium-low heat. Stir in brown sugar until well blended, then bring to a boil. Boil until sugar dissolves, 5 to 8 minutes. Carefully stir in cream (mixture may spatter) and pecans; reduce heat to low.
2. Make French toast: In a large shallow bowl, whisk together eggs, nutmeg, cinnamon, vanilla, and orange liqueur.
3. Heat a large cast-iron skillet or nonstick frying pan over medium heat. Melt 1 tbsp. salted butter in skillet, tipping pan to coat. Quickly dip bread, 1 piece at a time, into egg batter, coating both sides but not drenching. Lay bread in skillet and repeat with remaining bread. Cook until golden brown, 1 1/2 to 2 minutes; then flip, add remaining 1 tbsp. butter, and cook until second side is golden brown, 1 1/2 to 2 minutes more.
4. Transfer French toast to plates, top with some of warm toffee sauce, and serve remaining sauce on the side.
Make ahead: Sauce keeps up to 2 days, covered and chilled (reheat in a microwave oven).