Bring a pot of salted water to a boil. Add in potatoes and cook until potatoes are fork tender (about 20 minutes.) Drain the water. Return the potatoes to the pot and add in 4 tablespoons butter, milk and Greek yogurt. Mash. Set aside until you're ready to top the beef mixture with mashed potatoes.
Preheat the oven to 400 degrees.
If you use an oven safe Dutch oven you won't have to use a different dish to bake in.
Cook vegetables. Melt 3 tablespoons butter in a pot (Dutch oven or other skillet). Add onions, carrots and celery. (If using frozen vegetables add them with the onions). Cook until tender, stirring occasionally, about 8-10 minutes. Add in the seasonings (Tajin or chili powder, oregano and garlic powder and a pinch of salt and pepper). Break up the meat as it cooks, about 4-6 minutes.
Next, add in the flour, beef broth, Worcestershire sauce and tomato paste and stir to combine.
In an oven safe dish (or the Dutch oven you're using) layer the beef vegetable mixture at the bottom and spoon the mashed potatoes on top. Bake until brown around the edges and bubbly, about 25-30 minutes. Remove from the oven, spoon into dishes, allow to cool for a few minutes and serve.