Return the pot back to the stove; over medium heat & melt the butter. Stir in onion, celery & mushrooms; saute until vegetables are translucent, 5-6 minutes then stir in the garlic, flour & seasoning. Cook, stirring for 1 minute then add the shrimp & broth. Cook for several minutes, stirring until sauce thickens. Add 1/3 cup half & half & simmer gently for a minute or two. Remove from heat.