Preheat the oven to 350F/180C. Toss the rhubarb with sugar, lemon juice and zest and roast for 20 minutes until soft when pierced with a knife, then cool.
Meanwhile, in a food processor combine flour, ground almonds, cubed cold butter, sugar and baking powder and pulse until the mixture resembles fine bread crumbs, add an egg and pulse until just combined. (This could also be done in a bowl with a pastry blender.)
Take the pastry out of the food processor, it will be soft, wrap it in cling wrap and chill in the fridge for 20-30 minutes.
Roll out chilled pastry on floured surface and line an 9" loose-bottomed tart pan. Prick with a fork all over. Put it back in the fridge and chill for 20 more minutes.
Line the pastry with parchment paper, fill with baking beans and bake it blind for 15 minutes at 350 F/180 C. Then remove the beans and paper and bake for 7-10 more minutes. Make sure it doesn't get brown.
Beat together sugar, butter, almonds, eggs, almond extract, baking powder, salt and flour until creamy.
Divide the rhubarb and mash half of it with a fork. Spread mashed rhubarb over the base of the tart.
Then fill the tart with almond mixture.
Arrange the rest of rhubarb on top.
Bake in the oven for 40 minutes until golden and risen. Cool in the tart pan until just warm, then remove from the pan by pushing it out from the bottom.