Peel and finely slice the red onion. Pop it into a bowl and pour over the red wine vinegar. Add a pinch of salt and mix. Leave to marinate.
Thickly slice the mushrooms. Chop the tomatoes into quarters. Finely shred the sage leaves.
Warm an ovenproof frying pan over a medium heat for 1 min (20 cm is a good size pan for this dish). Add ½ tbsp oil. Tip in the lardons. Cook and stir for 4-5 mins till they start to crisp up and turn golden.
Add half the marinated onions to the pan, along with all of the mushrooms and half of the tomatoes. Cook for 4-5 mins, stirring occasionally, till the veg are soft.
While the veg are cooking, crack the eggs into a bowl. Beat together. Add a good crack of black pepper and most of the sliced sage. Pour this mix into the pan with the lardons and veg. Turn your grill on and set it to medium-high.
Cook the frittata on the hob for 3 mins till the base has set. Slide it under the grill and cook for 5 mins till bubbling and firm. Remove and leave to cool slightly.
Drain and rinse the beans. Tip them into a pan with the remaining marinated onions and tomatoes. Cook on a medium heat for 3-4 mins till warmed through. Tip in the spinach and stir together to just wilt it. Taste and add salt and pepper if you think it needs it. Serve the fritatta in slices alongside the warm bean salad.