In a food processor, place the tarragon and spinach and pulse 1 to 2 times to coarsely chop. Add the panko and pulse until completely combined with the tarragon and spinach (it should look like green bread crumbs). Transfer the mixture to a medium bowl and set aside.
In a small saute pan set over medium-high heat, add the olive oil and pancetta and cook until the pancetta is golden-brown and crispy, about 4 minutes. Remove the pan from the heat and add the lemon juice, sugar and salt, stirring with a wooden spoon until the sugar is dissolved. Add the parsley and set aside.
Pour enough grapeseed oil into 10-inch saute pan to fill it by 1/3 inch. Heat the oil over medium heat to 350 degrees F or until a few breadcrumbs sprinkled in the pan sizzle immediately but do not burn. Lightly beat the egg in a medium bowl. Add the oysters to the egg and toss gently to coat. One by one, lift them out of the egg and place them in the herb breadcrumbs. Again toss gently to make sure the oysters are coated all around, pressing gently to adhere.
Place the breaded oysters, one at a time, into the hot oil. You should be able to fry about 6 at a time. Fry until golden brown, about 1 minute. Flip the oysters and cook another 1 minute to brown on the other side. Remove from the oil with a slotted spoon and drain on a paper towel-lined plate. Repeat the process with the remaining oysters.
Place one oyster in each half shell and spoon some of the pancetta vinaigrette over the top. Serve warm.