Preheat the oven to 180C (350F) Gas Grease a wide baking dish with 1 tablespoon of the olive oil.
Halve the fennel bulbs lengthways and trim them, discarding the woody cores and reserving the leafy fronds.
Blanch the bulb halves for 8 minutes in a saucepan of boiling water. Drain and pat dry. Halve the (belpeppers lengthways and remove the pith and the seeds.
Heat 2 tablespoons of the olive oil in a frying pan/skillet. Sauté the onions and then add the garlic.
Transfer the onions and garlic to a large bowl and add the ricotta. Season with salt and pepper and add the pistachio nuts. Place 3–4 spoonfuls of the mixture in each of the (belpepper halves. Place the fennel on top so that it sits on the cheese mixture. Add more mixture around the edges of each, to fill.
Transfer the (belpeppers filled with cheese and fennel to the baking dish. Mix the passata/strained tomatoes and tomato purée/paste and season with parsley. Pour around the stuffed (belpeppers. Drizzle the remaining oil on top of the peppers, and sprinkle with a pinch of peperoncino.
Cover with foil and bake for 30 minutes in the preheated oven until golden brown and bubbling. Serve garnished with fennel fronds.