Set oven at 350.
Stir together flour, brown sugar, chopped nuts and melted butter.
Spread in 9x13 pan and bake for 20 minutes, stirring occasionally. Allow to cool and reserve 1/3 of crust mix for topping.
Dissolve package of jello in hot water. Add lemon just and sugar, then stir. Chill until slightly thickened using 2 cups of ice cubes. Once jello has slightly thickened, remove ice.
Whip can of chilled evaporated milk until very thick and pour jello mixture in. Pour onto crust and sprinkle reserved crumbs on top.
Chill in refrigerator until set.