Preheat oven to 350°F. Place 26 garlic cloves in small glass baking dish. Add 2 tablespoons olive oil and sprinkle with salt and pepper; toss to coat. Cover baking dish tightly with foil and bake until garlic is golden brown and tender, about 45 minutes. Cool. Squeeze garlic between fingertips to release cloves. Transfer cloves to small bowl.
Melt butter in heavy large saucepan over medium-high heat. Add onions and thyme and cook until onions are translucent, about 6 minutes. Add roasted garlic and 18 raw garlic cloves and cook 3 minutes. Add chicken stock; cover and simmer until garlic is very tender, about 20 minutes. Working in batches, puree soup in blender until smooth. Return soup to saucepan; add cream and bring to simmer. Season with salt and pepper.
Divide grated cheese among 4 bowls and ladle soup over. Squeeze juice of 1 lemon wedge into each bowl and serve.
Do ahead: Can be prepared 1 day ahead. Cover and refrigerate. Rewarm over medium heat, stirring occasionally.