Season the chicken breast fillets with salt, pepper, and paprika powder. Place in an ovenproof dish.
Cut the onions into half rings and finely chop the garlic. Heat the rapeseed oil in a large pan over medium-high heat. Add the onion rings and garlic, and cook for 5-7 minutes until softened. Add the peeled and chopped tomato, stir in the tomato paste, and cook for 3 more minutes, stirring occasionally.
Pour in the broth and sherry, stirring to deglaze the pan. Add the cream and bring to a gentle boil. Reduce heat to low and simmer for 2-3 minutes. If needed, dissolve the cornstarch in a small amount of water and stir into the sauce.
Pour the sauce over the chicken fillets. Bake in a preheated oven at 160°C (320°F) on the middle rack for 45 minutes, until the chicken reaches an internal temperature of 74°C (165°F).