1. Place horseradish in a small bowl and add vinegar. Cover and transfer to refrigerator; let chill for at least 2 hours and up to 1 week.

2. Place ground bacon in a large nonstick skillet over medium heat; cook until fat becomes foamy, 4 to 5 minutes. Add panko and season with salt and pepper; cook, stirring, until panko is golden brown, 5 to 6 minutes more.

3. Transfer bacon mixture to a large bowl and add zest of 1/2 lemon and thyme; toss to combine. Season with salt and pepper and spread mixture in an even layer on a baking sheet to cool.

4. Place butter in the bowl of an electric mixer fitted with the paddle attachment. Add 1/4 cup cooled bacon mixture, remaining zest of 1/2 lemon, 1/4 cup drained horseradish mixture, and 1/2 teaspoon pepper; mix until well combined. Transfer butter mixture to a large sheet of plastic wrap; shape into a log and wrap with plastic wrap to enclose. Refrigerate until firm.

5. Meanwhile, cut remaining slice of slab bacon in half lengthwise. Cut crosswise into thin strips. Place in a medium nonstick skillet and cook over medium heat until cooked through, but still slightly chewy. Remove from heat and set aside.

6. Preheat broiler. Place oysters on a baking sheet. Slice chilled butter crosswise into 1/4-inch-thick rounds; place a butter round on each oyster. Top with cooked bacon strips. Transfer baking sheet to broiler; broil until butter forms a dark, crispy crust, 3 to 4 minutes. Sprinkle with parsley and serve.