Stirring in peanut butter at the end rounds out the spiciness and adds body to this Thai soup, but fish sauce is the real secret here, used for its savory-salty punch.
In a medium saucepan, simmer stock, garlic, chiles, ginger, and fish sauce for 10 minutes.
Whisk in peanut butter. Add shrimp and cook until opaque, about 1 minute. Remove from heat.
To serve, fill bowls with baby spinach. Ladle soup over spinach and garnish with scallion.