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1
Whisk flour and semolina in medium bowl to blend. Add eggs and stir with wooden spoon until mixture clumps together; turn out onto floured surface and knead until dough is smooth, adding more flour by tablespoonfuls if dough is sticky, about 8 minutes. Form dough into ball; cover and let rest 1 hour. (Can be made 1 day ahead. Wrap in plastic; chill. Let stand at room temperature 1 hour before rolling out.)
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2
Heat oil in heavy large skillet over medium heat. Add onion and saute until soft, about 8 minutes. Stir in radicchio and wine; sprinkle with salt. Reduce heat to medium-low; cover and cook until radicchio is very soft, stirring frequently and adding water by tablespoonfuls if dry, about 40 minutes. Uncover and saute until all liquid evaporates, about 5 minutes. Remove from heat and cool. Mix in mascarpone cheese. Season with salt and pepper.
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3
Cut pasta dough into 6 equal pieces. Cover pieces with plastic wrap to keep from drying out. Set pasta machine to widest setting. Flatten 1 dough piece into rectangle. Run dough piece through machine. Fold in half crosswise (end to end) and run through again. Continue to run through machine, turning machine to narrower settings after 2 passes and dusting with flour as needed to keep from sticking, until pasta sheet is scant 1/16 inch thick (about last setting). Place pasta sheet on lightly floured work surface; cover with plastic to keep from drying. Repeat with remaining pasta pieces.
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4
Using sharp 3 1/2-inch-diameter cookie cutter, cut out 36 rounds from pasta sheets. Place 1 rounded teaspoon filling in center of 1 dough round. Brush edges of dough lightly with water. Fold dough over filling, forming half-moon and pressing to adhere. Press edges with tines of fork to seal tightly. Place ravioli in single layer on lightly floured baking sheet and let dry 30 minutes. (Can be prepared 6 hours ahead. Cover; chill.)
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5
Working in 2 batches, cook ravioli in very large pot of boiling salted water until just tender, stirring gently to prevent sticking, about 3 minutes. Drain. Transfer ravioli to bowl. Pour hot melted butter over; sprinkle with salt, pepper, and Parmesan. Toss gently and serve.
