Heat the oven to 190º C without fan, and with top and bottom heat. Grease an oven dish and line it with oven paper. Heat the broth in a pot, but don’t let it boil, just keep it warm.
Bring a pan of water and salt to the boil. Add the broccoli and cook until tender, about 5 minutes. You can also steam it to keep all the nutrients intact. Drain the broccoli and rinse with cold water to cool. Drain again and dry with some kitchen paper.
Add the butter to the frying pan, and when it has melted and begins to sizzle, add and sauté the chopped onions. Leave to cook, stirring occasionally, until the onion is tender, for about 5 minutes.
Sprinkle in the flour and stir well to distribute evenly. Add the warm broth and milk, increase the heat so it starts to boil, stirring all the time with a whisk to prevent lumps from forming.
Lower the heat so the sauce may cook over a medium-low heat and reduce, stirring from time to time.
After 5 minutes, take the sauce off the heat and add half the Cheddar cheese and the mustard. When the cheese has melted, add the broccoli and artichokes. Season to taste with salt, black pepper and the spicy sauce.
Put the vegetable mixture into the oven dish, sprinkle with the remaining Cheddar cheese and the Parmesan cheese. Bake in the oven for 20-30 minutes, until the cheese melts and starts to bubble.
Serve the dish immediately.