Grease a 13 x 9 x 2-inch baking pan; set aside.

For filling: In a medium saucepan combine rhubarb, the 1 cup granulated sugar and water.

Bring to boiling; reduce heat. Cover and simmer for 5 minutes.

Meanwhile, in a small bowl combine the 1/2 cup granulated sugar and the 2 tablespoons flour. Stir into rhubarb mixture. Cook and stir about 1 minute more or until thick.

Remove from heat; stir in vanilla extract and lemon zest. Set aside.

In a medium mixing bowl, combine the 1 1/2 cups flour, oats, brown sugar and baking soda. Using a pastry blender, cut in shortening until the mixture resembles coarse crumbs.

Stir in 1/2 cup pecans or walnuts. Reserve 1 cup of the crumb mixture.

Press remaining crumb mixture into the bottom of prepared pan.

Evenly spread rhubarb mixture on top. Sprinkle with reserved crumb mixture.

Bake in a 375 degrees oven for 30 to 35 minutes or until the top is golden brown. Cool on a wire rack. Cut into bars.