Preheat oven to 350°F.

Butter a 13x9 inch baking dish.

To began making the Topping: place the pecans in a food processor and pulse until coarsely chopped. Transfer to a cup and set aside.

Place the flour, sugar, and salt in the food processor. Scatter the butter on top. Pulse until the mixture resembles coarse crumbs.

To began making the Fruit: peel and core the apples. Cut the apples into 1/2 inch slices (making about 6 cups). Place them in a large bowl and toss with the lemon juice and vanilla.

Cut the rhubarb into 1/4 inch slices (making about 4 cups). Add the rhubarb to the bowl.

Place the lavender in a spice grinder with 1/4 cup sugar. Pulse until the lavender is finely ground. Transfer into small bowl.

Stir in the flour, cinnamon, and the rest of the sugar (1/2 cup) into the lavender mixture.

Sprinkle the spice mixture over the fruit and toss to combine.

Transfer the fruit mixture to the prepared baking dish.

Sprinkle the dish with the chopped pecans and crumb mixture.

Bake for 50 to 60 minutes, or until fruit is bubbling and the tipping is golden brown.

Serve warm.