Heat your oven to 200°C/Fan 180°C/Gas 6. Peel the potato, beetroot and carrots. Trim the courgette. Coarsely grate all of the veg into a large bowl (or see our tip below). Peel and thinly slice the onion. Add to the bowl with the veg.
Zest and juice the lemon. Add the zest and half the juice to the crème fraîche tub. Peel and crush the garlic in to the tub. Add the thyme leaves and plenty of seasoning. Mix well. Stir through the prepped veg.
Grate the cheddar. Add two thirds of it to the bowl. Tip all the creamy veg into a medium-sized dish and cover with foil. Slide into the oven and cook for 20 mins till the veg is beginning to soften. Remove the foil and cook for a further 20 mins.
Scatter the remaining cheese on top of the gratin after it has cooked for 40 mins. Slide back into the oven for a further 10 mins till golden and melted.
Add the remaining lemon juice to a small bowl. Whisk in ½ tbsp oil and season with salt and pepper. Toss through the salad leaves. Serve with the gratin.