Preheat oven to 450°. Combine cream, flour, 1/4 cup sugar, and diced apricots in a large saucepan. Bring to a simmer over medium heat and cook, whisking, until thick, about 3 minutes. Remove from heat, add kirsch, then whisk in egg yolks, one at a time.

Beat egg whites in a mixing bowl until foamy. Add cream of tartar and continue to beat until very stiff.

Butter a small soufflé dish (6 1/2" in diameter and 2 1/2" deep), dust with sugar, and spread 1/4 cup apricot mixture on bottom. Add about a third of the egg whites to remaining apricot mixture and gently fold whites and apricots together. Repeat, carefully folding in remaining egg whites in two more batches. Do not overmix. Spoon over apricot mixture into soufflé dish.

Make sure oven rack is low enough to allow soufflé room to rise as much as 2" above rim of dish, then bake soufflé until lightly browned on top, 12 - 15 minutes. Dust generously with confectioners' sugar and arrange sliced apricots on top. Serve immediately. (Soufflé will continue to cook as it rests, so recommend to guests that they start eating from the outside and work their way into the middle.)