One Pot Tomato Sausage Pasta

• 500 g sausages (chicken or pork), sliced
• 1 red onion, thinly sliced
• 2 cloves garlic cloves, crushed
• 1 red capsicum, thinly sliced
• 400 g tin diced tomatoes
• 300 g wholemeal penne pasta
• 3 cup chicken stock
• 2 tsp dried oregano
• 200 g spinach
• .5 cup parmesan cheese, grated