Buy the squid already cleaned if possible. If not, use a twisting motion to pull the tentacles away from the body. Avoid squeezing where the two parts of the animal meet, so as not to break the ink sac--a real mess. Remove the viscera and the beak from the tentacles by squeezing the point where the tentacles meet. Rinse everything, then set the tentacles aside.
Put the squid bodies and the onion into a small saucepan with barely enough water to cover. Bring the pot to a light boil and hold there for about three minutes. Strain the liquid and save. Remove the onions and reserve. Set the squid bodies aside to dry and cool.
Heat the butter in a saucepan until it bubbles. Sprinkle the flour into it and whisk as if you were making a roux, but stop after it thickens, before it begins to brown. Add the garlic and parsley and cook until the garlic is fragrant. Remove from the heat. Add the milk and whisk until the mixture has the texture of mashed potatoes.
Add the parsley, cayenne, salt, and crabmeat (or shrimp). With a rubber spatula, stir the mixture gently until the crabmeat is well distributed, but not broken.
Preheat the oven to 375 degrees. Using an iced tea spoon, stuff the squid bodies with the crabmeat mixture. Leave the last quarter-inch empty. Seal the opening with a toothpick. Lay the stuffed squid in a large baking dish, at most two layers deep. Cover with aluminum foil and bake for about 25 minutes.
While the squid are in the oven, bring the cream to a light simmer in a wide skillet, and reduce by about a third. Add the Romano cheese and stir until it melts into the cream. Stir in the white pepper and 1/2 cup of the reserved stock from poaching the squid.
Remove the squid from the oven and place them into the sauce. Agitate the pan back and forth to cover the squid with the sauce. Serve two to four squid per person, garnished with sliced green onions and a scant sprinkle of cayenne.
If you're inclined to fry the tentacles as a garnish, just dust them in corn flour seasoned with Creole seasoning and fry in 375-degree oil until golden brown.