To prepare filling: Combine rhubarb, 1 cup strawberries, sugar (or Splenda) and lemon zest in a large nonreactive saucepan. Let stand for 20 minutes (35 minutes if rhubarb is frozen). Bring to a simmer over medium-low heat. Cook, stirring often, until the rhubarb is tender but still holds its shape, 5 to 8 minutes.
Meanwhile, stir cornstarch and water in a small bowl until smooth. Stir into the simmering fruit. Cook, stirring constantly, until the mixture is clear and very thick, about 1 minute. Transfer to a bowl. Place a piece of plastic wrap directly on the surface and refrigerate until chilled.
To prepare crust & assemble tart: Preheat oven to 350F. Coat a 9-inch tart pan with a removable bottom with cooking spray.
Spread oats in a small baking dish and bake, stirring occasionally, until toasted, 10 to 15 minutes. Let cool. Place the oats in a food processor and process until finely ground.
Combine milk and vanilla in a small bowl. Whisk the ground oats, flour, sugar, lemon zest, baking powder and salt in a large bowl. Drizzle oil onto the dry ingredients and stir with a fork or your fingers until crumbly. Use a fork to stir in the milk mixture, 1 tablespoon at a time, until the dough just comes together.
Turn the dough out onto a floured work surface and knead 7 to 8 times. Roll the dough out to an 11-inch circle, dusting with flour if necessary. Transfer to the prepared pan, pressing to fit. Trim the edges.
Line the tart shell with a piece of foil or parchment paper and fill with pie weights or dried beans. Bake the tart shell until set, 10 to 12 minutes. Remove weights and foil or paper and bake until lightly browned, 8 to 12 minutes more. Cool in the pan on a wire rack.
Shortly before serving, spread the strawberry-rhubarb filling evenly into the tart shell. Arrange the remaining 2 cups strawberries decoratively over the filling.
Heat jelly in a small saucepan over low heat, stirring constantly. With a pastry brush, glaze the strawberries with the jelly.
Ingredient note: In the EatingWell Test Kitchen, sucralose is the only alternative sweetener we test with when we feel the option is appropriate. For nonbaking recipes, we use Splenda Granular (boxed, not in a packet). For baking, we use Splenda Sugar Blend for Baking, a mix of sugar and sucralose. It can be substituted in recipes (1/2 cup of the blend for each 1 cup of sugar) to reduce sugar calories by half while maintaining some of the baking properties of sugar. If you make a similar blend with half sugar and half Splenda Granular, substitute this homemade mixture cup for cup.When choosing any low- or no-calorie sweetener, be sure to check the label to make sure it is suitable for your intended use.