How to Make It
In a medium saucepan, combine the rhubarb, sugar and elderflower syrup and bring to a simmer. Cook over moderately low heat, stirring occasionally, until thick, 50 minutes to 1 hour. Scrape the jam into a heatproof bowl and let cool to room temperature. Stir in the lemon juice.
Preheat the oven to 350°. Butter a 10-inch fluted tart pan with a removable bottom. In a bowl, whisk the oats with the barley flour, barley flakes, brown sugar, baking powder, salt and thyme. Using your fingers, blend the cubed butter into the oat mixture; blend in the water until well incorporated. Firmly press two-thirds of the mixture evenly over the bottom and up the side of the tart pan.
Spread 1 1/2 cups of the rhubarb jam in the tart shell; reserve the remaining jam for another use. Pinch the remaining dough into large crumbs and scatter over the jam. Bake the tart for about 40 minutes, until the topping is deep golden. Transfer to a rack to cool completely.
In a bowl, whisk the skyr with the sugar, lemon zest and rose water, if using. In another bowl, beat the cream to stiff peaks. Fold the whipped cream into the skyr. Serve the tart with the whipped skyr.
Elderflower syrup is available at many specialty food stores and from kalustyans.com. Alternatively, substitute St-Germain elderflower liqueur.