Pat chicken dry and season both sides generously with salt and pepper. Heat oil in large skillet over medium-high heat until shimmering. Add chicken and cook 5-6 minutes per side until golden and internal temperature reaches 165°F. Remove and set aside, covered loosely with foil.

Reduce heat to medium. Add butter to the same skillet (don't wipe it out). Once melted, add minced garlic and sauté 1 minute, stirring constantly, until fragrant but not browned.

Sprinkle flour over garlic and butter, stirring constantly to create a roux. Cook 1-2 minutes while stirring to eliminate raw flour taste. Mixture should be smooth and slightly golden.

Gradually whisk in chicken broth, adding slowly at first while whisking constantly to prevent lumps. Bring to a simmer and cook 3-5 minutes until thickened to coat the back of a spoon.

Stir in heavy cream if using. Taste and adjust seasoning with salt and pepper. Keep warm over very low heat, stirring occasionally.

While chicken cooks, bring large pot of salted water to boil. Add cubed potatoes and boil 12-15 minutes until fork-tender. Drain thoroughly.

Return drained potatoes to hot pot. Add butter, milk, and shredded cheddar cheese. Mash until smooth and creamy (or leave slightly chunky if preferred). Season with salt and pepper to taste.

To serve, place generous scoop of mashed potatoes on each plate. Slice rested chicken and arrange on top of or beside potatoes. Pour hot garlic gravy generously over chicken and potatoes. Garnish with fresh parsley or chives.