For the pastry - Mix the flour and paprika. Add the chilled butter, using your fingertips rub it into the flour till it resembles coarse breadcrumbs.
Add water, a couple of tablespoons at a time, just until the dough comes together to form a ball (you can also use a food processor for this, I prefer to do it by hand). Do not knead the dough too much; just work it till it holds together.
Flatten the pastry into a disk; wrap in cling film and stick it in the fridge to chill for 30 minutes (more if the dough has not firmed up).
To check if the dough is ready, press it lightly with your fingertips, it should leave an imprint but should not sink into the dough easily.
Take the pastry out of the fridge and on a lightly floured surface knead just a little to bring it together. Roll out the dough on a lightly floured surface roughly 3 mm thickness.
Carefully lift the rolled out pastry and gently press it into the bottom and up the sides of the tart tin. Trim the edges and patch any cracks with the remaining dough.
Place the crust in the fridge to chill for 15 minutes. This dough will easily make two 8 inch tarts; I used moulds with a removable bottom.
Preheat the oven to 200 degree Centigrade. Using a fork, prick all over the bottom of the crust, then place baking paper on top of the crust and fill it with baking weights or beans or rice (to prevent the crust from rising).
Bake in the oven for 15 minutes. Take it out of the oven and remove the baking paper and beans.
Thinly slice the tomatoes (cherry tomatoes and grape tomatoes can be added whole). Arrange the slices in the tart shells, sprinkle the thyme and add crumbled goat cheese.
Bake again till the tomatoes have softened (10-15 minutes). Top with mustard greens and serve.