Microwave, boil, or steam your carrots and potatoes until tender-crisp. The meat doesn't cook long enough for your veggies to cook fully through unless you chop them SUPER SMALL. You have been warned!
Season your meat with salt and pepper. Combine butter, garlic, mustard, and sage together in a small mixing bowl. Mix until well combined.
Add carrots and potatoes into 6-quart slow cooker. Place meat on top of veggies and surround with onion. Rub herb butter on meat. Surround with thyme sprigs.
Cover and cook on low heat for 3-4 hours, or until internal temperature reaches 140-145F (my slow cooker turns off when it reaches 140F!) Move to cutting board, cover with aluminum foil, and allow to rest for 10-15 minutes.
Slice and serve with additional fresh herbs if desired!
