Preheat the oven to 400°F (205°C).
Heat the olive oil or butter in a large skillet or Dutch oven over medium heat.
Add the diced onion and sauté for 5–7 minutes, or until softened and translucent.
Add the ground beef. Cook while breaking it into small crumbles until browned and almost completely cooked through.
Stir in the minced garlic and cook for approximately 1 minute.
Add the Worcestershire sauce, tomato paste, dried parsley, rosemary, thyme, salt, and black pepper.
Stir well and cook for another 1–2 minutes to develop the flavors.
Sprinkle the gluten-free flour evenly over the beef mixture.
Stir thoroughly until the flour is incorporated and no dry clumps remain.
Slowly pour in the beef broth while stirring.
Add the frozen corn, peas, and carrots.
Reduce the heat to low and simmer for approximately 10 minutes, stirring occasionally, until the vegetables are tender and the gravy has thickened.
Taste and adjust the seasoning if necessary.
Transfer the beef mixture to a 9-by-9-inch or similarly sized casserole dish and spread it evenly.
Spoon the mashed potatoes over the filling and carefully spread them all the way to the edges.
Use a fork to create ridges and peaks across the mashed potatoes. These raised areas will become especially golden during baking.
Bake the Gluten Free Shepherd’s Pie for 20–25 minutes, or until the filling is bubbling around the edges and the potato topping is lightly golden.
For additional browning, broil briefly at the end while watching the casserole carefully.
Remove from the oven and let rest for approximately 10 minutes before serving.
Garnish with chopped chives or parsley if desired.