You can use store-bought shrimp broth to save time. To make your own, place the shrimp shells and heads in an Instant Pot and cover with water (about 10-12 cups). Pressure cook on high for 3 hours. Strain and use 4 cups for this recipe.
Heat the butter and flour in a dutch oven over medium heat, whisking constantly until a light brown roux forms (about 5-10 minutes).
Add the tomato paste, garlic and shallot and cook for 1-2 minutes.
Very SLOWLY pour in the sherry while whisking constantly to ensure lumps don't form.
Do the same with the stock. Then, stir in the tarragon, bay leaf, coconut sugar, salt, paprika and crab boil.
Bring to a boil, turn down the heat and let it simmer, uncovered, for 10 minutes.
Stir in the cream and bring it back to a simmer for 10 minutes.
Remove from heat, add the prawns, cover and let sit for 10-15 minutes. (The prawns will cook through from the heat of the broth).
Serve with croutons and parsley.