Preheat oven to 400F. Grease a 12-cup muffin pan with 2 tbsp butter. Dust cups with bread crumbs. Tap sides of pan to evenly coat, then shake out excess crumbs. Melt remaining 1/4 cup butter in a medium saucepan over medium heat. Add garlic. Stir until fragrant, 1 min. Whisk in flour and mustard powder. Gradually pour in milk, whisking constantly until a smooth, thick paste forms. Continue whisking constantly 2 min. Transfer to a large bowl.
Separate egg yolks from whites, placing yolks in a small bowl and whites in a large bowl. Whisk yolks with tarragon and nutmeg, then whisk into warm-milk mixture. Crumble in cheese and gently stir, leaving some of the cheese in lumps.
Add salt to egg whites, then beat on high using an electric mixer, until soft peaks form when beaters are lifted, 2 to 3 min. Stir 1/3 of whites into cheese mixture. Gently fold in remaining whites just until no white streaks remain.
Spoon mixture into prepared muffin cups, filling right to top. Set muffin pan in a large roasting pan. Place on centre rack of oven. Pour enough hot water into roasting pan to come halfway up sides of muffin pan. Bake until soufflés have risen and are lightly golden, 15 to 17 min.
Whip cream in a large bowl until it holds soft peaks. In another bowl, whisk mascarpone until fluffy. Whisk in lemon zest. Stir in half of whipped cream until evenly mixed, then fold in remaining whipped cream.
Remove muffin pan from roasting pan. Place on a wire rack. Let stand 3 min. Run knife around inside edge of cups. Turn out soufflés and serve immediately with mascarpone cream and salmon.
Nutrition (per serving)
Wrap cooled soufflés with plastic and refrigerate up to 1 day. To heat, line a baking sheet with foil, then lightly butter. Leave soufflés at room temperature 15 min. Warm in a 400F oven until puffy and golden, 8 to 10 min.