In a large pan heat oil over medium. Add chicken. Sprinkle with salt and black pepper. Adjust heat so chicken sizzles but doesn't burn. Cook 5 to 8 minutes, undisturbed, until browned and releases easily from pan.
Meanwhile, for sauce, in a medium bowl combine tomato paste and vinegar. Add 1 1/2 cups water, the sage, and crushed red pepper. Sprinkle with salt and pepper. Whisk until sauce is smooth.
Turn chicken. Cook 5 minutes more or until browned and releases easily from pan. Scatter mushrooms and garlic into pan. Pour sauce over mixture. Shake pan once or twice to combine. Bring to boiling; reduce heat. Simmer 20 minutes, uncovered, or until chicken just separates from the bone, carefully shaking pan a couple of times. Season to taste.