Generously butter 9-1/2-inch diameter deep-dish pie plate. Place 1 phyllo sheet on work surface (cover remaining pieces with plastic wrap, then with clean damp towel). Brush phyllo sheet with butter and fold in half lengthwise. Brush folded surface with butter. Cut in half crosswise.
Place 1 phyllo rectangle, buttered side down, in prepared pie plate, covering bottom and letting pastry overhang 1 section of edge by 1/2-inch. Brush top of phyllo in pie plate with butter.
Place second phyllo rectangle in pie plate, covering bottom and letting pastry overhang another section of edge by 1/2-inch; brush with butter.
Repeat process with remaining 4 phyllo sheets, making certain entire surface of edge is covered to form crust. Fold overhang under to form crust edge flush with edge of pie plate.
Brush crust edges with butter. (Can be prepared 4 hours ahead. Cover and refrigerate.) Preheat oven to 350F. Whisk yolks and mustard in medium bowl to blend.
Beat in eggs, half and half, cream, salmon, onions and chopped dill. Season to taste with salt and pepper. Pour into prepared crust. Bake until center is set, about 50 minutes. Transfer to rack. Cool.
Garnish with dill sprigs and serve slightly warm or at room temperature.
NOTE: Purchased phyllo pastry is used here instead of a regular pie crust for quick and easy assembly. This tart is best when served at room temperature.