To prepare crepes, weigh or lightly spoon 3 ounces (about 2/3 cup) all-purpose flour into dry measuring cups; level with a knife. Combine all-purpose flour, 1 cup milk, buckwheat flour, salt, and egg in a medium bowl, stirring with a whisk until smooth. Cover and let stand at room temperature 30 minutes.
Heat a 9-inch crepe pan or nonstick skillet over medium heat. Coat pan with cooking spray. Pour about 1/4 cup batter into center of pan; quickly tilt pan in all directions so batter covers pan with a thin film. Cook about 1 minute. Carefully lift edge of crepe with a spatula to test for doneness. Turn crepe over when it can be shaken loose from pan and underside is lightly browned; cook 30 seconds. Place crepe on a towel; cool. Repeat procedure five times with remaining batter. Stack crepes between single layers of wax paper or paper towels to prevent sticking.
Preheat oven to 300°.
To prepare filling, heat a large nonstick skillet over medium heat. Coat pan with cooking spray. Add ham and garlic to pan; cook 3 minutes or until garlic is tender and ham is lightly browned, stirring frequently. Add mushrooms; reduce heat to medium-low. Cover and cook 10 minutes or until mushrooms start to soften, stirring occasionally. Add broth, parsley, and pepper; cook, uncovered, 10 minutes or until liquid evaporates and mushrooms are tender. Remove from heat.
Spoon 1/3 cup ham mixture into the center of each crepe. Top each serving with 1 tablespoon cheese; fold sides over. Place filled crepes in an 11 x 7–inch baking dish. Cover with foil; bake at 300° for 20 minutes or until thoroughly heated.
To prepare sauce, melt butter in a medium saucepan over medium-low heat. Add shallots; cook 5 minutes or until tender, stirring frequently. Sprinkle 1 tablespoon all-purpose flour into pan; cook 2 minutes, stirring constantly. Slowly add 1 1/2 cups milk, stirring constantly with a whisk until smooth; bring to a simmer over medium heat. Reduce heat, and simmer 10 minutes or until sauce thickens, stirring frequently. Remove from heat. Strain sauce through a fine mesh sieve into a bowl; discard solids. Rinse and dry pan; place over low heat, and return strained sauce to pan. Stir in 1/2 cup cheese and remaining ingredients; stir until cheese melts. Serve sauce with crepes.