Heat oil or butter in a pan and saute diced onion with salt on high heat until glassy / translucent look for about 5 minutes
Add diced celery then saute it for 5 minutes then add sliced mushrooms. Stir fry until the mushrooms collapse about half of their size for about 5 minutes.
Lower the heat to medium then add broccoli and stir fry it for 5 - 10 more minutes. Frozen broccoli needs a bit more time to get ready but about 10 -15 minutes it should be ready to go.
Mix in cream, tuna then pour everything into a casserole dish.
Sprinkle grated cheddar and Parmesan on top. Some like to add crushed pork rinds on top but this completely changes the dish's flavor portfolio and it's not really encouraged.
Put it under the broiler on maximum heat until the cheese gets a golden brown color for about 10 - 15 minutes.