Preheat the oven to 350° F. Line a 9x13 inch baking dish with parchment paper. Add the butter to a skillet set over medium heat. Allow the butter to brown, until it smells toasted, about 2-3 minutes. Remove from the heat. In a food processor, pulse together the pretzels and brown sugar until the pretzels are mostly ground with a few chunks left. Add the rye flour, vanilla, and browned butter. Pulse to combine. Press the dough into the prepared pan. Bake 12 minutes, until golden. Let cool.To make the cream. Whip the ricotta in a food processor until smooth and creamy. Add the powdered sugar and pulse to combine. The cream can be kept in the fridge for up to 3 days. To assemble, spread the cream over the crust and dollop the jam over the cream, gently swirling it into the cream. Arrange the strawberries on top. In a small bowl, combine the sugar and lavender (if using) and sprinkle over the berries. Slice and serve immediately. If making ahead, add the berries just before serving. The tart keeps in the fridge for 1-2 days.