Melt 1 tbsp butter in a soup pot and sauté reserved shrimp shells for 1-2 minutes until pink.
Add stock, onion, celery, and bay leaf; simmer for 10 minutes to build the shell broth.
Strain the broth, discarding shells and vegetables, and set the liquid aside.
In the same pot, melt remaining butter and sauté garlic for 30 seconds until fragrant.
Stir in tomato paste, then pour the strained broth back into the pot.
Bring to a gentle simmer, stir in cream if using, and season to taste.
Add raw shrimp, remove from heat, and let cook gently for 2-3 minutes until just opaque.
Stir in fresh herbs and lemon juice, then serve immediately.