Whisk together the flour, salt and dried active baking yeast. Stir in the warm water and 3 tablespoons of the olive oil to form a soft dough. Turn the dough out onto a lightly floured board, and knead, until smooth and elastic, about 10 minutes.
Cover the dough and let rise in a warm place until doubled in volume, about 35 minutes.
Preheat the oven to 220 C / Gas 7. Oil a pizza tray.
Roast the red pepper for 10 minutes in the preheated oven. Peel, seed and slice the pepper into thin slices. Meanwhile, heat the remaining 3 tablespoons olive oil over medium heat in a frying pan, and cook and stir the onions until golden, about 5 minutes.
Roll out the pizza base on a lightly floured board, and transfer the base to the prepared pizza tray. Spread the pizza base with passata. Add the onions evenly over the top and sprinkle with the grated Gruyère cheese. Arrange the pepper slices over the cheese and sprinkle with the herbes de Provence.
Bake in the preheated oven until the pizza base is golden and the cheese is bubbly, about 10 minutes.