In a medium nonstick sauté pan, heat butter over medium-high heat until butter melts. Add olive oil. Add onion, celery, carrots, lamb, garlic salt, and pepper to pan. Cook, stirring frequently, until lamb is browned and vegetables are tender, 3 to 5 minutes.
Add broth, thyme, and barley. Bring to a boil. Reduce heat to low, cover, and cook for 15 minutes, stirring occasionally. Add Marsala.†
In a small bowl, combine cornstarch and water, stirring until smooth. Add to lamb stew, stirring and cooking until mixture is thickened and broth is clear, 2 to 3 minutes.
Bake pastry shells according to package directions.
Fill pastry shells with warm lamb stew.
Garnish each serving with a fresh thyme sprig, if desired.
Serve immediately.
Notes
*For testing purposes, we used Pepperidge Farm Puff Pastry Shells. †At this point, mixture can be covered and stored in the refrigerator for a day. When ready to serve, heat to boiling, and then reduce heat to low. Thicken with cornstarch mixture.
Recipe by TeaTime Magazine at https://teatimemagazine.com/irish-lamb-barley-stew-in-patty-shells-recipe/