Slow Cooker Cream of Mushroom Soup

1/2 cup butter
1 medium onion, chopped
2 garlic cloves, minced
6 cups sliced mushrooms (about 24 oz.)
1 package (32 oz.) vegetable broth
1 cup dry white wine
1 teaspoon dried tarragon
1 1/2 teaspoons salt
1 1/2 teaspoons coarse ground pepper
1 cup heavy cream
1/4 cup cornstarch

1. In a large skillet over medium-high heat, melt butter. Add onion and garlic. Cook 3 to 4 minutes, or until onions are clear.
2. Add mushrooms and cook until tender. Add wine and cook 2 minutes.
3. Transfer mixture to slow cooker crock. Add vegetable broth, tarragon, salt and pepper.
4. Cover slow cooker and cook on HIGH for 2 to 2 1/2 hours or LOW for 3 to 4 hours.
5. In a small bowl, whisk cream and cornstarch until smooth. During last 30 minutes, stir cream mixture into soup in crock. Stir until smooth. If using LOW heat, turn to HIGH. Cover and cook until thickened.
6. Remove half of soup mixture with sliced mushrooms. Transfer remaining mixture to a blender or use a hand blender to blend mixture in crock until somewhat smooth. Combine with reserved mushroom soup mixture and serve.