Heat oven to 350F. Butter bottom, sides and rim of a 9-inch quiche pan, tart pan or pie plate.
Using a strainer, lightly sift soy powder over bottom, sides and rim of pan; set aside.
In a 10-inch skillet, melt butter over medium heat. Add diced scallions and minced garlic; cook, stirring occasionally, until scallions begin to soften, about 1 minute.
Add diced bacon, and cook stirring occasionally, just until bacon begins to brown, about 3 minutes. Add sliced mushrooms, basil, thyme, sage, oregano and rosemary; cook, stirring occasionally, until mushrooms are softened, 5 to 7 minutes. Set aside.
In a medium bowl, whisk eggs, heavy cream and mustard together until thoroughly blended. Evenly spoon mushroom mixture on bottom of prepared pan.
Evenly sprinkle cheese over mushroom mixture. Evenly pour egg mixture over top. Arrange tomato slices over tart.
Bake until tart is slightly puffed and golden brown and a knife inserted near center comes out clean, 25 to 30 minutes. Serve warm or at room temperature, garnished with thyme sprigs, if desired.