Melt butter in a large stockpot over medium heat. Stir in flour to make a smooth paste, and cook for about 3 minutes, stirring constantly. Mix in the onion, celery, and garlic; season with salt and pepper. Continue to cook and stir for about 4 minutes. Gradually whisk in the half and half cream
In a heavy-bottomed pot, melt butter, add flour, and stir gently until blended; do not burn or let it darken. Add milk gradually, stirring all the while, then add half and half, stirring gently while blending. Add corn and green onions, and cook for a few minutes until tender.