Skinny Shepherd's Pie

I made this lighter version of shepherd's pie with mashed cauliflower instead of mashed potatoes. It is perfect comfort food for a cold winter day.

1 large head of cauliflower
1 lb lean ground beef
1 large onion, diced
1 tsp olive oil
1 tbsp Worcestershire sauce
1 tbsp ketchup
3/4 lb frozen mixed vegetables (peas, corn, carrots, and green beans)
1 cup 2% milk shredded cheddar cheese

Preheat oven to 400 degrees.
Place head of cauliflower in a pot of water and boil until cauliflower is soft.
In the meantime, saute onions in olive oil until tender. Add the ground beef to the onions and let it brown. Then add the Worcestershire sauce and ketchup. Remove the beef mixture from heat and stir in frozen vegetables.
When cauliflower is soft, drain most of the water and use an immersion blender or mixture until smooth.
Transfer the beef mixture to a casserole dish. Layer the mashed cauliflower, and then the cheese. Bake in 400 degree oven lightly brown.