Lightly coat the slow cooker with nonstick cooking spray and set aside.
Heat the oil in a skillet over medium-high heat for about 2 minutes.
Add the onions and cook for 5 minutes, stirring occasionally, until softened.
Add the ground beef along with the herbs and seasonings. Cook for 6 to 8 minutes, breaking up the meat and stirring occasionally, until browned. Drain any excess grease.
Stir in the garlic and Worcestershire sauce and cook for 1 minute until fragrant.
Add the tomato paste and flour, stirring until fully incorporated and smooth.
Pour in the broth and add the frozen vegetables. Bring the mixture to a boil, then reduce to a simmer and cook for 5 minutes, stirring occasionally, until the sauce thickens. Set the filling aside.
To make the mashed potato topping, place the potatoes in a pot and cover with water. Bring to a boil.
Reduce the heat to a simmer and cook for 10 to 15 minutes, or until the potatoes are fork-tender.
Drain the potatoes and return them to the pot. Let them rest for 1 minute.
Mash the potatoes with the remaining ingredients until smooth and creamy.
To assemble the shepherd's pie, spoon the beef mixture evenly into the bottom of the Crockpot.
Spread the mashed potatoes evenly over the meat mixture.
Cover with the lid and cook on low for 5 to 6 hours, or until bubbly and heated through.