Heat butter in large saucepan; cook onion and garlic, stirring, until onion softens. Add flour; cook, stirring, until mixture bubbles. Add mushrooms; cook, stirring, until mushrooms are just tender.
Add tomato paste and wine to mushroom mixture; bring to a boil. Reduce heat, simmer, uncovered, until liquid is reduced by half. Add the water and stock, return to a boil. Reduce heat; simmer, uncovered, 30 minutes. Stir in thyme.