Heat a large pot over medium heat and add the ground beef. Cook until browned, breaking it up with a spoon, then drain any excess fat.
Add the onion, garlic, and mushrooms to the pot. Cook for 5 minutes until the onion softens and the mushrooms release their liquid.
Stir in the carrots, potatoes, beef broth, thyme, salt, and pepper. Bring the mixture to a boil.
Reduce the heat to low, cover the pot, and simmer for 20 minutes until the potatoes and carrots are tender.
Turn off the heat and stir in the frozen peas. Let the stew sit for 3 minutes so the peas warm through without losing their color.