Drain the oysters in a colander over a bowl, reserving 1 cup oyster liquid.
Heat a large Dutch oven coated with cooking spray over medium-high heat. Add onion, celery, bell pepper, and garlic; saute 5 minutes. Lightly spoon flour into a dry measuring cup; level with a knife. Add flour to onion mixture; cook 1 minute, stirring constantly. Gradually add reserved oyster liquid and broth, stirring with a whisk until blended. Stir in thyme and bay leaf, and bring to a boil. Add oysters, green onions, and milk; cook 3 minutes or until edges of oysters curl. Gently stir in crabmeat, salt, and 1/4 teaspoon black pepper; cook 1 minute or until thoroughly heated. Discard bay leaf. Sprinkle with freshly ground black pepper.