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Place the cut strawberries in a bowl and toss with 2 tablespoons of brown sugar, set aside.
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Place the rhubarb pieces, lemon juice and 1/4 cup of brown sugar in a saucepan and heat stirring until the rhubarb pieces soften but have not completely dissolved (about 5-7 minutes.) You want to still see the shapes of the rhubarb pieces. Remove from heat, mix together with the sugared strawberries and set aside to cool.
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Place the chopped hazelnuts in a naked small skillet over medium-low heat. Stir until toasted, remove from heat and set aside to cool.
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Preheat the oven to 350 degrees F.
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Grease and flour a 10-inch tart form.
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In a separate bowl, whisk together the flour, baking powder, and salt.
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In a mixer cream together the butter and brown sugar. Beat about 3 minutes.
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Mix in the egg, vanilla extract, and lemon zest.
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Stop the mixer and scrape down the sides of the bowl. Turn the mixer back on and beat until thoroughly combined.
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Slow the mixer down and add half the dry ingredients and mix to combine.
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Add the other half of the dry ingredients and the toasted hazelnuts and mix only until streusel forms.
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Lightly press 2/3 of the mixture (use a kitchen scale for precise measurement) into the prepared tart form.
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Place the pan in the oven and bake for 10 minutes. Remove from oven.
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Use a slotted spoon to fill the tart with the strawberry rhubarb mixture, leaving out any excess liquid.
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Distribute the remaining streusel over the top. Gently press it down with your palms.
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Replace the pan in the oven and bake for another 20 minutes.
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Remove from oven. Let cool. for 15-20 minutes. Cut and serve.