To prepare stew, combine 3/4 cup onion, tomato paste, thyme, bay leaves, and beer in a large zip-top plastic bag. Add beef; seal and marinate in refrigerator 4 to 24 hours, turning bag occasionally. Remove beef from bag, reserving the marinade.
Cook potatoes and carrot in boiling water 7 minutes; drain. Rinse in cold water; drain.
Heat a Dutch oven coated with cooking spray over medium-high heat. Add 3/4 cup onion and garlic; sauté 2 minutes. Add beef; cook 3 minutes or until browned. Lightly spoon 1/4 cup flour into a dry measuring cup, and level with a knife. Sprinkle 1/4 cup flour over the beef mixture, stirring to coat. Cook 2 minutes. Gradually add reserved marinade, stirring well, and bring to a boil. Reduce heat, and simmer 20 minutes. Add potatoes, carrot, spinach, 1/4 teaspoon salt, and pepper; cook 20 minutes or until potatoes are tender. Discard thyme and bay leaves. Remove from heat, and cool slightly. Spoon into a 10-inch deep-dish pie plate coated with cooking spray. Top with oysters.
To prepare crust, lightly spoon 1 cup flour into a dry measuring cup, and level with a knife. Combine 1 cup flour and 1/4 teaspoon salt in a bowl, and cut in butter with a pastry blender or 2 knives until mixture resembles coarse meal. Sprinkle surface with ice water, 1 tablespoon at a time, tossing with a fork until moist and crumbly (do not form a ball).
Press mixture gently into a 4-inch circle on heavy-duty plastic wrap; cover with additional plastic wrap. Chill for 15 minutes. Roll dough, still covered, into an 11-inch circle. Place dough in freezer for 5 minutes or until plastic wrap can be easily removed.
Preheat oven to 350°.
Remove 1 sheet of plastic wrap; fit the dough over top of beef mixture. Remove top sheet of plastic wrap. Fold the edges under beef mixture. Brush surface with milk. Pierce top of dough with a fork; bake at 350° for 40 minutes or until the filling is bubbly and crust is browned.