Morels, the wild mushrooms that are a member of the truffle family, have a distinctive smoky, nutty essence. If you can't find fresh wild morels, substitute other seasonal wild mushrooms, such as oyster mushrooms. Slice the spring onions and cook the filling mixture while the pastry chills.
1 cup all-purpose flour (about 4 1/2 ounces)
2/3 cup cake flour (about 2 2/3 ounces)
1/2 teaspoon salt
3 tablespoons chilled butter, cut into small pieces
To prepare pastry, lightly spoon flours into dry measuring cups; level with a knife. Combine flours and 1/2 teaspoon salt in a medium bowl. Cut in butter with a pastry blender or 2 knives until mixture resembles coarse meal. Combine sour cream and juice in a bowl, stirring well. Stir sour cream mixture into the flour mixture. Sprinkle surface with 3 tablespoons ice water, 1 tablespoon at a time; toss with a fork until moist and crumbly (do not form a ball).
Gently press mixture into a 4-inch circle on plastic wrap; cover. Chill 20 minutes. Slightly overlap 2 sheets of plastic wrap on a slightly damp surface. Unwrap and place chilled dough on plastic wrap. Cover dough with 2 additional sheets of overlapping plastic wrap. Roll the dough, still covered, into a 12-inch circle (about 1/4-inch thick). Place dough in freezer 5 minutes or until plastic wrap can be easily removed. Remove top sheets of plastic wrap; place dough, plastic wrap side up, onto a baking sheet lined with parchment paper. Remove remaining plastic wrap.
Preheat oven to 400°.
To prepare filling, thinly slice white parts of onions, reserving green tops. Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add sliced onions to pan; sauté 15 minutes or until golden. Add mushrooms to pan; cook 5 minutes or until moisture evaporates. Remove from heat. Slice green tops of onions. Add sliced tops, 1/4 teaspoon salt, and pepper to pan; stir to combine. Spread onion mixture evenly over dough, leaving a 2-inch border; sprinkle cheese evenly over onion mixture. Fold edges of dough toward center, pressing gently to seal (dough will only partially cover onion mixture). Place pan on bottom rack in oven; bake at 400° for 25 minutes or until browned.