Shuck the oysters into a bowl and refrigerate. Discard the shells.
Cut into the top of the sea urchins and then around the sides with a strong pair of scissors. Carefully spoon out the orange or yellow roe and reserve it in a small bowl in the refrigerator. If you're serving the stew in the sea urchin shells, scrub them thoroughly and dry them in a 200 degree oven.
Blanch the spinach leaves in boiling water for 30 seconds, rinse them in a colander under cold running water, squeeze out the excess water and reserve.
Warm the oysters in their liquid in a saucepan over medium heat until the liquid barely begins to simmer at the edge of the pan and the oysters begin to curl around the edges, about 3 minutes.
With a slotted spoon, transfer the hot oysters to a clean kitchen towel or clean napkin so any grit will pull off the oysters and cling to the towel. Strain the oyster-cooking liquid through a fine strainer into a clean saucepan.
Add the lemon juice and 4 tablespoons of the heavy cream to the oyster liquid and simmer gently for about 5 minutes, until the mixture has the consistancy of cold heavy cream. Meanwhile, simmer the remaining tablespoon of cream in a small pan until it thickens. Season with salt and pepper and add the spinach. Stir for a minute over medium heat to heat the spinach. Arrange the spinach in a small mound in the center of each soup plate or sea urchin shell.
Gently stir the chives, oysters and sea urchin roe into the oyster sauce and heat gently--without boiling--for about 30 seconds. Arrange the oysters and roe around and over the spinach. Make sure everyone gets 4 oysters. Spoon over the sauce. Serve immediately.