For the pastry crust combine the flour, herbs de Provence, and salt in the bowl then add butter and quickly cut it into the flour until the mixture resembles a coarse meal with either your hands, pastry cutter, or in a food processor. Add a tablespoon of ice water at a time and mix until a dough comes together adding more water if needed. Shape it into a thick disk, wrap it in plastic, and refrigerate for at least 2 hours before rolling.
Next, caramelize the onions by adding the butter and olive oil to the pan and heating over medium heat. Season with some salt and turn the heat down to medium-low stirring frequently to prevent burning. Right before the onions are perfectly soft and caramelized, add a big slash of wine, balsamic vinegar, or some broth to glaze the pan and scrape all the flavor. Remove the pan from the heat and allow the onions to fully cool.
Pull the dough from the fridge to roll, lightly flour the dough, and counter then roll to about a 1/8 inch thick then place over a tart pan and press into the mold then prick all over with a fork. If you have the time place the tart pan with the dough back into the fridge to re-chill for 20-30 minutes. Add a sheet of parchment paper over the dough with dried beans or blind baking beans. Bake at 350 F for 12-15 minutes or until slightly golden.
While the pastry crust is baking whisk together all the filling ingredients but the radish slices, and goat cheese. Pour the filling into the par-baked crust then dollop with goat cheese crumbles and the slices of radish all over the top.
Place back in the oven to bake until the filling has set and doesn’t giggle and the crust is golden
Let the tart cool slightly before cutting and enjoying!