Preheat a medium nonstick sauté pan over medium-high heat, and add olive oil. When oil shimmers, add meat, onion, celery, garlic salt, and ⅛ teaspoon pepper. Cook, stirring, until meat is well browned and vegetables are tender. Drain fat, and transfer meat mixture to a bowl. Set aside.

In the same pan, melt 2 tablespoons butter over medium heat. Add flour, whisking and cooking until a roux forms and mixture is very pale brown. Add chicken broth, whisking until mixture is thickened and smooth. Add ¼ teaspoon salt, remaining ⅛ teaspoon pepper, and Kitchen Bouquet. Add meat mixture, mixed vegetables, and thyme, stirring to incorporate. Reduce heat to a simmer. When heated through, remove mixture from heat, cover, and keep warm.

In a small saucepan, cover potatoes with water, add remaining ¼ teaspoon salt, and bring to a boil over medium heat. Cover, and cook until tender when pierced with a fork, approximately 15 minutes. Drain well, and add milk and remaining 1 teaspoon butter. Beat at medium speed with a mixer until smooth. Add egg, beating to combine.

Divide warm meat mixture among 6 (2-ounce) broiler-safe ramekins. Set aside.

Transfer whipped potatoes to a piping bag fitted with a large open-star tip (Wilton 1M). Pipe decorative rosettes onto meat mixture.

Place ramekins on a rimmed baking sheet, and place under a hot broiler briefly to lightly brown potato rosettes, being careful not to burn.

Serve immediately.