Heat your oven to 200°C/Fan 180°C/Gas 6. Lightly oil a large ovenproof dish. Remove the outer leaves and tough stalk from the cauliflower. Break it into evenly sized florets. If it’s very big, just use half (see our tip). Give the florets a quick rinse. Pat dry.
Warm 1 tbsp oil in an ovenproof frying pan. Add the cauliflower florets. Season. Cook and stir over a high heat for 6-8 mins till golden. Lift the cauli out of the frying pan and set aside. (If your frying pan isn’t ovenproof, transfer the cauli to an ovenproof dish.)
Peel and slice the onion. Strip the sage leaves from the stalks and slice them. Add ½ tbsp oil to the frying pan. Add the onion and sage. Stir and fry over a medium heat for 5 mins till soft. Add the cauliflower back to the frying pan (or tip the sage and onion into the dish with the cauliflower).
Crack the eggs into a large jug. Season with salt and pepper. Beat with a fork or whisk. Pour the beaten eggs over the veg.
Put the ovenproof frying pan onto a baking tray. Bake in the centre of the oven for 20-25 mins till golden and puffed up. Pop a skewer in the middle – if it comes out clean the frittata is ready.
While the frittata bakes, make a salad. Squeeze the juice from half the lemon into a large bowl. Add 1 tbsp oil. Season. Whisk together with a fork.
Trim the dry ends from the fennel, then finely slice it. Halve the tomatoes. Add them all to the lemon dressing. Add the salad leaves. Toss to coat everything. Serve with the frittata.