Preheat the oven to 400°F.

In a nonstick skillet over medium heat, saute celery, scallions, button mushrooms, and garlic in 2 tablespoons of broth; about 8 minutes or until soft, adding more broth if needed.

Place the sauteed veggies in a food processor with cooked shrimp and all other remaining ingredients, except the portobello mushrooms; pulse the processor until mixture is finely chopped.

Place spoonfuls of the mixture on the underside of the portobello mushrooms.

Bake on a nonstick baking sheet for about 30 minutes.