Heat the oven to 350 F and lightly butter a 2-quart baking dish.
Heat the oil in a large skillet over medium heat. Cook the ground beef, undisturbed, for 4 minutes. Continue cooking, stirring and breaking up the beef until it is no longer pink, about 4 to 6 minutes longer.
Drain the beef, transfer it to a plate, and set it aside.
Return the skillet to medium heat and melt the butter. Add the onion and cook for 4 to 5 minutes, until translucent. Add the garlic and cook for 1 minute longer.
Stir in the tomato paste and flour and cook, stirring, for 2 minutes.
Add the Worcestershire sauce, stock, and herbs and cook while stirring until the mixture has thickened.
Add the browned beef and thawed frozen peas and carrots to the skillet. Taste and add salt and pepper as needed.
Transfer the beef mixture to the prepared baking dish.
Spread or pipe the potatoes over the beef filling. Sprinkle with cheddar cheese if desired.
Bake the cottage pie for 25 to 30 minutes or until the top is browned and the beef filling is bubbling.