Preheat the oven to 180°C (160° fan-forced). Place beef and kidneys in a large snap lock bag or dish, add 2 tablespoons of the flour, season and rub to coat well.
Heat 1 tablespoon oil in a large ovenproof casserole dish over medium-high heat. Add the beef and kidneys and brown, in batches, for 2-3 minutes, adding a little extra oil as required. Set browned meat aside on a plate.
Reduce heat to medium and add remaining tablespoon of oil. Add onion, celery and carrot and cook for 3-4 minutes, stirring occasionally. Sprinkle in remaining flour and stir until vegetables are coated. Return beef, kidneys and any juices to dish. Gradually pour in stock and add tomato paste, stirring well. Add thyme and stir until mixture boils. Cover dish, place in oven and cook for 2 hours or until the beef is very tender, stirring occasionally. Stir in spinach. Cool for 10 minutes, if time allows.
Meanwhile, cook cauliflower rice in the microwave according to packet instructions. Place cauliflower in a large bowl, add milk, mustard and cheese, season and stir well to combine.
Increase oven heat to 200°C (180° fan-forced). Divide beef mixture between six 1-cup capacity dishes. Top with cauliflower mixture and sprinkle with extra cheese. Cook for 10-15 minutes or until top is golden. Serve pies with peas, tomato chutney and extra thyme.
Swap the kidneys for 200g brown mushrooms, thickly sliced, if preferred. Add mushrooms for last half hour of cooking time.
Swap cauliflower for potato or sweet potato for an alternate pie topping.
Any remaining pie filling will freeze well for later use.