Put the flour, yeast and 1 tsp salt in a large bowl, ensuring the yeast and salt don’t touch. Pour in 350ml lukewarm water and the olive oil, and bring together with a spoon to form a wet, elastic dough. If using a stand mixer, knead for 5 mins or until the dough stops sticking to the side of the bowl. If kneading by hand, knead for 10 mins on a lightly floured surface.
Put the dough in a lightly oiled bowl, cover with a clean, damp tea towel and leave to rise for 1 hr or until doubled in size.
Line a 20 x 30cm roasting tin with a piece of crumpled baking parchment. Cut the dough in half and press, as lightly as possible, one half into the roasting tin, stretching it out to fill the tin. Cover with the basil, mozzarella and pine nuts, then spread out the other half of the dough and place on top. Cover and leave to rise for 1 hr or until light and puffy.
Meanwhile, heat oven to 200C/ 180C fan/gas 6. Once risen, press indents with your fingers into the top of the dough. The dough is ready if the indents stay; if they bounce back, leave the dough for longer. Fill the indents with pesto, then sprinkle with a little flaky sea salt and drizzle with a little olive oil.
Bake on the top shelf of the oven for 25-30 mins, without opening the oven door, until golden.
Transfer to a wire rack and leave to cool completely. Take to your picnic whole and slice into 10-12 sandwiches when serving.