Prep ingredients: Trim ends from celery, then finely chop. Peel and finely chop 2 large garlic cloves. Trim ends from 2⁄3 of the scallions, then thinly slice. Scrub potatoes and cut into 1⁄2-inch pieces. Pick and finely chop 1⁄3 of the thyme leaves (save rest for own use).

Sauté aromatics: Heat 11⁄2 tablespoons oil in a medium pot over medium-high. Add celery, garlic, 3⁄4 of the sliced scallions, and 1 teaspoon salt, and cook until softened, 2– 3 minutes. Stir in 11⁄2 tablespoons flour and cook, stirring, about 1 minute.

Add liquid: Stir in 3 cups water and all of the shrimp broth concentrate, cover and bring to a boil.

Add potatoes: Add potatoes and 2⁄3 of the chopped thyme (save rest for step 6) cover, and bring to a boil. Reduce heat to medium and simmer, partially covered, until potatoes are tender when pierced with a knife, about 10 minutes.

Add shrimp: Cut each shrimp into 3 pieces and add to the pot. Cover and simmer over low until pink, 2–3 minutes. Stir in mascarpone and peas and cook until heated through, about 1 minute.

Finish chowder & serve: Season chowder to taste with salt and pepper. Serve in bowls garnished with remaining thyme and sliced scallions.