3/4 cup shredded Gruyere cheese or Swiss cheese, divided
Select saute setting on a 6-qt. electric pressure cooker and adjust for medium heat; add oil. When oil is hot, cook and stir prosciutto until crisp, about 3 minutes. Remove from pan with a slotted spoon. Add onion to pressure cooker; cook and stir until tender, 4-5 minutes. Press cancel.
In a large bowl, whisk egg substitute, milk, mustard and pepper. Stir in bread, half the cheese, and onions. Reserve 2 tablespoons cooked prosciutto for topping; stir remaining prosciutto into bread mixture.
Transfer to a greased 1-1/2-qt. baking dish. Wipe pressure cooker clean. Place trivet insert and 1 cup water in pressure cooker. Cover baking dish with foil. Fold an 18×12-in. piece of foil lengthwise into thirds, making a sling. Use the sling to lower the dish onto the trivet.
Lock lid; close pressure-release valve. Adjust to pressure-cook on high for 20 minutes. Let pressure release naturally for 10 minutes; quick-release any remaining pressure. Using foil sling, carefully remove baking dish. Sprinkle with remaining cheese and prosciutto; cover and let stand 10 minutes.