Preheat oven to 400ºF (200ºC). Loosen the oysters from the shell and set aside in a medium bowl. Arrange the rock salt in a baking dish and firmly press the shells into the salt.
In a medium sauté pan over medium heat, add the butter and allow to melt. Add the shallots and sauté until soft and translucent, about 4 minutes. Add the garlic and sauté an additional 15 seconds.
Add the spinach, parsley, onions and tarragon and stir until the greens have wilted, about 4 minutes. Stir in the lemon juice, cayenne, paprika, salt and pepper followed by the cream.
Add the oysters back into the shells. Top with a spoonful of the greens mixture followed by the breadcrumbs and Parmesan cheese.
Bake until the breadcrumbs are lightly browned, about 15 minutes. Garnish with chives and serve immediately.
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Notes
Use an oyster knife to gently loosen the oysters from their shells before baking—this makes them easier to eat once cooked and prevents the topping from slipping off.