Wrap the fish in the flavoured papers and seal on a sauté pan. Finish off cooking in the oven at 200°C for 8 minutes. Meanwhile, sauté the fennel, shallots and garlic with a little olive oil until softened. Add the stock and simmer by half, followed by the cream and bring to the boil. Whisk in the butter and remove off the heat. Cook the spaghetti in a pan of salted boiling water for 2 minutes than toss in the sauce. Add in the cherry tomatoes, spinach and leeks. Toss together well, season and serve the pasta in deep pasta bowls with the fish on top.