Prick potatoes all over with a fork. Microwave potatoes on high, turning once, until tender, about 15 minutes. Remove from microwave, let cool slightly, then peel and mash with milk and butter. Season with salt and pepper.
In a skillet over medium heat, warm olive oil. Add onion and cook, stirring, until translucent, 3 minutes. Add garlic and cook 30 seconds longer. Add beef, increase heat to medium-high and cook, breaking up chunks, until meat is cooked through and beginning to brown, 8 minutes. Stir in tomato paste, Worcestershire, thyme and 1/2 tsp. each salt and pepper. In a small cup, whisk cornstarch with 1 cup water. Add to skillet and cook, stirring, until liquid thickens slightly. Remove from heat; transfer to slow cooker.
Layer vegetables on top of meat; top with potatoes. Cover, turn slow cooker to low and cook for 3 hours.