In a shallow dish, stir 2 cups room-temperature water with 1 tablespoon kosher salt to make a brine. Add the 8 ounces sea scallops and let them soak for 10 minutes.
Remove the scallops from the brine and pat them as dry as possible with paper towels. (Extra surface moisture will make the pan spit and prevent a good sear.) Lightly season both sides with kosher salt.
Heat a medium stainless-steel or cast-iron skillet over high heat until very hot. Add 1½ tablespoons vegetable or canola oil and swirl to coat.
Carefully add the scallops to the hot pan in a single layer, leaving space between them. Do not move them. Cook until a brown crust forms on the bottom, about 1 to 3 minutes depending on your heat.
Flip each scallop and cook on the other side until a crust forms and the center is just almost opaque, about 1 to 2 minutes. Remove the scallops to a plate, leaving any browned bits and juices in the pan.
Reduce the heat to medium-low and add 1 tablespoon salted butter to the pan. When the butter has melted, add the 2 minced garlic cloves and 1 teaspoon dried oregano. Cook, stirring, until the garlic is lightly browned and fragrant, about 30 seconds.
Pour in ½ cup heavy whipping cream and ¼ cup milk, then add 3 tablespoons freshly grated Parmesan cheese. Bring to a gentle simmer and cook for about 2 minutes, stirring occasionally and scraping the browned bits from the bottom of the pan.
Add 2 cups chopped fresh spinach, ¼ cup chopped sun-dried tomatoes (packed in oil), and ¼ teaspoon kosher salt. Cook, stirring gently, until the spinach is wilted, about 2 minutes.
Return the scallops to the pan and warm them in the sauce for about 30 seconds, spooning the sauce over the scallops so they are coated and heated through.
Serve immediately spooned over rice or couscous.