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1
Whisk 1/4 cup plus 1 tablespoon warm water and yeast in small bowl; let stand until yeast is dissolved, about 10 minutes.
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2
Mix flour, nigella seeds, and salt in medium bowl. Add yeast mixture and stir with wooden spoon until mixture clumps together. Add butter; mix until incorporated. Turn dough out onto lightly floured work surface and knead until smooth and elastic, about 5 minutes. Divide dough into 6 equal pieces; roll each into ball. Place on baking sheet; cover with plastic wrap. (Can be prepared 1 day ahead. Keep covered and refrigerate.)
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3
Heat oil in large nonstick skillet over medium-high heat. Add onion wedges to skillet and cook without turning until beginning to brown, about 7 minutes. Carefully turn onions over; reduce heat to medium and continue to cook until onions are tender and brown, occasionally stirring gently, about 10 minutes. Add thyme and honey; cook 1 minute. Stir in vinegar. Seasonto taste with salt and pepper. Remove skillet from heat. Cool. (Can be prepared 1 day ahead. Cover and refrigerate.)
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4
Let dough rise in warm draft-free area until doubled, about 1 hour. Whisk creme fraiche and egg in small bowl to blend. Stir in 1/2 teaspoon salt and 1/4 teaspoon pepper.
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5
Preheat oven to 450u00b0F. Sprinkle 2 rimmed baking sheets lightly with semolina flour. Roll out each dough ball to 5-inch round. Transfer 3 rounds to each prepared baking sheet. Let rest 10 minutes. Pierce dough rounds all over with fork. Spread 1 generous tablespoon creme fraiche mixture over each dough round, leaving 1/4-inch plain border. Arrange onions evenly atop each; drizzle lightly with creme fraiche mixture (mixture may spill over edges of dough).
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6
Bake flatbreads until crusts are puffed and golden, about 12 minutes. Transfer to plates; sprinkle with chives.
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7
*Available at some supermarkets, and at specialty foods stores and Italian markets.
