To prepare crust: Using finger tips blend flour, salt, and butter until mixture resembles coarse meal. (Butter should be in tiny lumps: this makes crust flaky.) Form a well in center of dough. Add egg and water. Using finger tips, mix just until dough comes together and forms an easy ball. Flatten dough into thick disk. Wrap in wax paper. Chill for at least 1 hour. On floured surface, roll out dough to 1/8-inch thickness. Press into tart pan (about 10 inch size). Cut of excess dough 1/2 inch beyond edge of pan. Fold over to form sides of tart, so they are double thick at bottom. Using a fork, poke holes in bottom.
To make filling: In large pan, heat butter and oil over medium heat. Add onions. Cook slowly, stirring often until onions are a dark golden brown. Remove from heat. Cool 20 minutes. Mix egg and cream into onions. Stir well. Add nutmeg, salt and pepper. (Be careful with nutmeg; a little goes a long way.)
To bake tart: Fill tart shell with onion mix. Bake in 350-degree oven for 45 minutes, or until top and crust are dark golden brown. Serve slices hot out of the oven or at room temperature. It is delish!
