Preparation Before beginning, gather all ingredients and equipment. Clean the mushrooms with a paper towel, avoiding water which can make them soggy. Finely chop the portobello mushrooms into uniform pieces about 1/4 inch in size. Measure out all sauce ingredients, as the cooking process requires quick additions. Let chicken breasts come to room temperature for even cooking.

Mushroom Sauce Base Heat the soybean oil in a large saucepan over low heat. Gentle heating is crucial for proper sauce development and prevents the roux from burning. While the oil heats, combine the water and condensed mushroom soup in a separate container. Whisking until completely smooth. Set this mixture aside within easy reach of your cooking area.

Once the oil is warm (test with a small piece of mushroom - it should gently sizzle), add your fine chopped portobello mushrooms to the pan. Stir constantly with a wooden spoon or heat-resistant spatula for exactly one minute. This brief cooking releases the mushrooms' flavor while maintaining their texture. You'll notice the mushrooms becoming slightly darker and fragrant.

Gradually sprinkle the flour into the mushroom and oil mixture while whisking continuously. This is the foundation of your sauce - ensure there are no lumps by whisking vigorously. Keep the heat low and continue whisking for about two minutes to cook out the raw flour taste without allowing it to brown. The mixture should look like wet sand at this stage.

Develop the Sauce Slowly pour in the prepared water and mushroom soup mixture while whisking constantly. This gradual addition prevents lumps from forming. Continue whisking as the mixture comes to a gentle boil - you'll notice it beginning to thicken. Reduce heat to maintain a gentle simmer and cook for exactly 5 minutes, stirring occasionally. The sauce should coat the back of a spoon when ready.

Season the Sauce Add the garlic powder, granulated sugar, salt, and white pepper to the thickened sauce. Stir thoroughly to ensure even distribution of seasonings. Let the sauce simmer for an additional 2 minutes to allow the flavors to meld. Taste and adjust seasonings if needed, remembering that the cheese will add saltiness later.

Prepare the Chicken While the sauce is finishing, brush both sides of your room-temperature chicken breasts with oil. Ensure each breast is evenly coated. Heat your grill or grill pan to medium-high heat (around 375°F). You should hear a sizzle when you place your hand a few inches above the surface.

Grill the Chicken Place the oiled chicken breasts on the preheated grill. Cook for 6-7 minutes on the first side without moving them - this ensures proper grill marks and caramelization. Flip only once and cook for an additional 5-6 minutes. Or cook until the internal temperature reaches 165°F, test with meat thermometer.

Add the Cheese Layer Transfer the grilled chicken to a baking dish large enough to hold all pieces without overlapping. Place slices of Jack cheese on each chicken breast, ensuring even coverage. Place the dish under a preheated broiler set to low, about 6 inches from the heat source. Watch carefully - this should take only 2-3 minutes until the cheese is melted and slightly bubbling.

Final Assembly Remove the baking dish from the broiler. Pour your prepared mushroom sauce over the chicken, ensuring each piece is generously covered. Sprinkle freshly grated Parmesan cheese over the entire dish. Return to the broiler for just 1 minute to melt the Parmesan slightly. Finish with a sprinkle of freshly chopped parsley.