Preheat your oven to 375F and move the rack to the top third of the oven.
Add the butter to a large skillet and melt over medium-high heat, then add the onions and sauté for 5 minutes.
Add the beef to the skillet and cook, breaking it up with your spoon as you go along, until just browned (about 6-8 minutes).
Reduce the heat to medium and stir in the Italian seasoning, tomato paste, and flour. Cook (stirring constantly), for about one minute.
Stir in the beef broth, Worcestershire sauce, and salt, and scrape up any browned bits from the bottom of the skillet. Let the liquid thicken up a bit (about 1 minute).
Stir in the frozen mixed vegetables and heat them through for about 3 minutes, stirring often. Do not let the mixture get too dry (add a splash more broth if it does dry out).
Transfer the meat mixture to a 9x13 casserole dish, smooth it in an even layer, and then arrange the tater tots on top. Or, if your skillet is oven-safe, large, and deep, you can make the casserole right in the skillet like I did (I used a 12" Le Creuset). I arrange the tater tots from the outside in when doing it in a skillet.
Bake, uncovered, for 40 minutes or until hot and bubbly. Optional: broil for a few minutes (watch it carefully) to brown the tater tots a bit more.