Preheat oven to 400°. In a cold, large cast-iron or ovenproof skillet, cook bacon over medium heat, stirring occasionally, until fat has rendered and bacon is crisp, 13 to 15 minutes. Drain on paper towels, then transfer to a large bowl.

Increase heat to medium-high. Cook mushrooms, stirring occasionally, until starting to brown, about 5 minutes. Add thyme, half of garlic, and 1 teaspoon salt and cook, stirring, until fragrant, about 1 minute. Remove from heat and stir in parsley; season with pepper.

Add mushroom mixture, cream cheese, and panko to bowl with bacon and stir until combined. Fold in gouda cheese.

Pat turkey dry with paper towels. Using a sharp knife, gently cut a deep incision into thickest side of breast, working from top to bottom and being careful not to cut all the way through; season all over with oregano, 2 teaspoons salt, and ½ teaspoon pepper. Using a spoon, fill turkey breast with mushroom mixture until full, packing in with spoon. Cut 5 (12") pieces twine and snuggly tie around breast.

Into same skillet, pour 1 tablespoon oil and place turkey in skillet. Drizzle remaining 1 tablespoon oil over turkey.

Roast turkey until an instant-read thermometer inserted into thickest part registers 160°, 45 to 50 minutes.

Transfer turkey to a cutting board and tent with foil.

In a medium bowl, whisk stock and cornstarch. Heat same skillet over medium-high heat. Cook remaining garlic, stirring, until fragrant, about 1 minute. Add stock mixture to skillet and scrape up any browned bits from bottom of skillet. Cook, stirring occasionally, until liquid is reduced by half, about 5 minutes. Add cream and lemon juice and reduce heat to low. Simmer, stirring occasionally, until sauce is thickened, about 5 minutes more.

Slice turkey and transfer to a large platter. Top with lemon cream sauce and parsley.