Preheat oven to 200*C.
In a skillet, drop in your olive oil and onions and cook the onions slowly until soft.
While they are cooking, chop up the cauliflower and cook that in a separate pan - either in boiling water or steam it or microwave it or whatever you do to get your cauliflower soft.
Add the beef mince and crushed garlic and cook until the beef is, well, cooked.
Throw in the tomato paste and Worcestershire Sauce and combine. Then throw in your mushrooms. THEN add the spinach and wish that you'd chosen a bigger skillet like I didn't.
Cover and cook slowly until the spinach is more or less wilted down.
While this is happening, you can mash up the cauliflower with a potato smasher. If you really wanted to, you could add butter, but I didn't think it really needed it. I threw in the chives here.
In a glass baking dish, spoon your meat mixture and pat it down so it's quite compact. Then smear the mashed cauliflower on top.
Bake in the oven for about 15 minutes or so. If you REALLY wanted to, you could put some shredded cheese on top and grill it until bubbly. Comfort food!