Heat the grill to high. Put the bacon rashers on a baking tray lined with foil and grill for 5 mins on each side until crisp. Leave to cool, then roughly chop.
Heat the oil in a large saucepan or high-sided frying pan over a high heat. Season the chicken, then brown on both sides. Sprinkle over the oregano and add the garlic, then stir for another minute. Crumble in the stock cube and add 250ml water.
Bring to a simmer, cover and reduce the heat to low-medium. Cook until the chicken is tender, about 15-20 mins, topping up with more water if the pan looks dry. Transfer the chicken to a board using tongs, then shred using two forks. Return the shredded chicken to the pan with the sauce.
Whisk the flour into the cooking juices in the pan, turn up the heat and keep stirring until you have a smooth paste that clings to the chicken. Add 500ml water and the cream, then simmer, stirring occasionally for 8-10 mins, or until the sauce has thickened.
Meanwhile, cook the pasta in a large pan of boiling salted water for about 10 mins until it is just cooked but retains some bite. Drain.
Add most of the parmesan, the mustard, spinach and half the chopped bacon to the sauce, then season to taste, if needed. Stir until the spinach has wilted, then stir in the cooked pasta.
Tip the mixture into a large baking dish. Mix the breadcrumbs with the remaining parmesan and scatter over the top along with the tomatoes and remaining chopped bacon. The unbaked dish can now be frozen for up to two months (see below).
To cook straightaway, heat the oven to 200C/180C fan/gas 6. Bake for 35-40 mins, or until the top is golden and crunchy and the sauce is bubbling.