Place large, heavy-bottomed pot on stovetop over medium heat. When pot is warm, add 1 tablespoon refined coconut oil and swirl or tilt pot as needed to distribute oil across entire surface. Continue heating pot until oil is melted, hot, and shimmery.
When oil is hot, add 1 cup thinly sliced white onion, 2 cloves garlic, 2 tablespoons sliced red jalapeño pepper, 3 ¼-inch-long slices fresh galangal, 1 lemongrass stalk, and 2 teaspoons red Thai curry paste to pot. Stir to incorporate, then sauté ingredients 5 minutes or until onions have softened. Stir ingredients frequently to prevent sticking or burning.
When onions have softened, pour 4 cups low-sodium chicken broth into pot and increase temperature under pot to medium-high. Stir to incorporate broth, making sure to scrape bottom of pot to loosen any ingredients that may have stuck during sauté.
Bring chicken broth to rolling boil. Once boiling, immediately reduce heat to medium-low. Simmer mixture, uncovered, 30 minutes, stirring occasionally.
After 30 minutes, use slotted spoon to remove large garlic pieces, onion slices (optional), lemongrass pieces, and slices of galangal. Discard aromatics.
Remove pot from heat and let mixture cool slightly. Pour 4 cups unsweetened, full-fat coconut milk into cooled mixture and stir to incorporate, then return pot to stovetop over medium-low heat.
Add 8 ounces white mushroom caps to pot. Stir to incorporate, then simmer mixture 3 to 5 minutes or until mushrooms have softened.
Add 1 ½ tablespoons fish sauce, 1-2 tablespoons coconut sugar, and 2-3 tablespoons fresh lime juice. Stir to incorporate additions, then simmer mixture 2 minutes. After 2 minutes, taste mixture and adjust fish sauce, coconut sugar, and lime juice as desired.
When satisfied with flavor of mixture, add 1 pound shrimp to pot. Stir gently to incorporate, then simmer 1 to 2 minutes or until shrimp are opaque, lightly pink, and curled into slight "J" shape.
When shrimp is cooked through, immediately portion soup into serving bowls. Garnish with thinly sliced green onions and chopped fresh cilantro if desired. Serve immediately.