Heat your oven to 200°C/Fan 180°C/Gas 6. Tip the flour into a mixing bowl. Finely grate in half the cheddar. Chop in the cold butter. Use your fingers to rub the ingredients together till they resemble coarse breadcrumbs.
Carefully crack the egg and separate the yolk from the white. Stir the yolk into the flour mixture with 3 tbsp cold water till you have a soft dough. Wrap in cling film and put in the fridge for 15 mins to rest. Keep the egg white for later.
While the dough rests, peel and thinly slice the onions and garlic. Pour ½ tbsp oil into a frying pan and set on a mediumlow temperature. Add the veg. Cook and stir for 5 mins till soft. Stir in 1 tbsp of the vinegar and a little salt and pepper. Take off the heat.
Lay the rested pastry on a sheet of greaseproof paper. Roll into a rough circle, a little larger than a dinner plate. Slide onto a flat baking tray.
Scatter the onions on top of the pastry, leaving a gap of about 3cm at the edge. Grate the remaining cheddar on top. Thinly slice the courgettes and arrange on top – it will look full but they will cook down. Season well with salt and pepper. Fold in the edges of the pastry to create freestanding sides. Brush the edges with the egg white.
Bake in the oven for 20 mins. Scatter the pine nuts over the top and bake for 5 more mins till the pastry is golden brown.
While the galette bakes, make a dressing in a bowl by combining ½ tbsp olive oil with the remaining vinegar, a pinch of salt and pepper and the thyme leaves. Slice the tomatoes and tumble with the rocket in a bowl. Spoon the dressing over the top. Toss to mix.
Serve thick slices of the warm galette with a pile of the dressed salad.