For the croutons: In a large bowl, toss the butter, sage and bread together to coat well. Spread in a single layer on baking sheet and bake for 15 minutes at 375°, turning once halfway through. Set aside.
For the chowder: In a large saucepot, melt butter and add the onions, celery, and carrots. Cook until tender. Add the garlic and fresh thyme. Sauté for 3 more minutes.
Add the potatoes and chicken broth. Simmer for 15 minutes or until potatoes are soft.
Use an immersion blender and blend the ingredients to your desired consistency.
Add the lemon juice and zest, half & half, salt, pepper, and sherry. Stir well.
Top with croutons and fresh thyme.