Fill a large pot with water and set your sous vide circulator to 122F. Let it come fully up to temperature.
Season the sardine fillets with salt and pepper. Place them with olive oil and thyme sprigs in a sealed bag.
Submerge the bag in the water bath and cook for 25 minutes.
Melt the butter in a saucepan, add the shallot, and cook for 2-3 minutes until softened.
Add the broth and reduce by half. Stir in the cream and simmer for 3-4 minutes until slightly thickened.
Remove from heat and stir in tarragon and lemon juice. Season with salt.
Remove the sardines from the bag and pat dry gently.
Plate the sardines and spoon the warm tarragon cream sauce over top.