Set rack on lowest level of oven and preheat to 400° F. Butter and flour a 2-quart soufflé dish or a 9×13 inch glass baking dish. If using a soufflé dish, extend the height of the dish with 4 inch wide collar made from folded strip of aluminum foil tied around the dish. Also butter and flour foil.
In a medium skillet, cook scallion in 1 tablespoon butter over medium heat, about 1 minute just to soften. Add crab meat and cook, stirring 1-2 minutes. Layer in bottom of prepared dish.
In a medium saucepan, melt remaining 2 tablespoons butter over medium-high heat. When foamy, add flour and cook, stirring 1 minute. Blend in tomato paste. Gradually whisk in warm milk. Bring to a boil, whisking until sauce is thickened and smooth, 1-2 minutes. Season with cayenne and salt. Remove from heat and whisk in beaten egg yolks. Stir in Parmesan cheese.
In a large bowl, beat egg whites and cream of tarter to stiff peaks. Fold sauce into whites with gentle wide strokes. Pour over crab meat.
Place dish in oven and immediately reduce heat to 275°F. Bake 25 minutes, until soufflé is puffed and golden on top and slightly jiggly in center.