Preheat oven to 350°F. Cook pasta according to package directions until al dente; drain and set aside. Add one teaspoon olive oil to large sauté pan and place over medium heat. When oil is hot, add pancetta and cook until golden brown on both sides, about 3-4 minutes. Remove pancetta with spoon and transfer to bed of paper towels to drain. Set aside.
Add onion to same sauté pan and cook until translucent, about 3-5 minutes. Place garlic and thyme over the onions and cook until fragrant, about 1 minute. Add butter and melt. Once melted, mix in flour and cook 1-2 minutes. Increase heat to medium-high and whisk in milk until mixture has thickened, about 4- 6 minutes. Add a few teaspoons of warm milk mixture to bowl with egg yolks and whisk. Add egg-milk mixture back to pan and mix until completely combined.
Turn off heat and add Dijon mustard, 1 cup Grand Cru and 1 cup parmesan. Add salt to taste and mix well. In large bowl, add pancetta, pasta and cheese mixture. Toss together until pasta is evenly coated. Transfer to a 13”x 10” casserole dish and sprinkle top with remaining cheese and freshly cracked pepper. Transfer to oven and bake 12-15 minutes, until top is melted. Serve immediately.