Combine brown rice and its water in a saucepan, bring to a simmer, cover and cook over low heat for 35 to 40 minutes until tender. Rest for 5 minutes and spread on a plate to cool slightly.
Cut the tops from peppers and remove seeds and membranes without piercing the bottoms. Finely chop the edible flesh from the tops.
Mix raw beef, cooked rice, onion, garlic, chopped pepper tops, 1/2 cup tomato sauce, Worcestershire, salt, thyme and pepper. Fill peppers loosely without packing the mixture down.
Pour remaining tomato sauce and broth into a 6-quart slow cooker. Stand peppers upright in the sauce, cover and cook on low for 6 hours until tender and the center of every filling reaches at least 160°F.
Lift peppers carefully with two large spoons and rest for 5 minutes. Spoon the hot cooking sauce over each to serve.