1. Heat oil in a 12-inch skillet over medium-high heat.
2. Place flour in a medium-sized bowl. Whisk egg and water together in another bowl.
3. Season cutlets with salt and pepper, then dredge 3 cutlets in flour. Dip in egg mixture until evenly coated and place in the hot skillet. Cook, turning once, until golden brown and cooked through, about 5 minutes. Transfer to a serving platter and repeat with remaining cutlets, adding more oil to the skillet if needed.
4. Add mushrooms to the skillet and season with salt and pepper. Cook, stirring often, until they release liquid and begin to brown, about 3 minutes. Add cream and cook, stirring, until slightly reduced, about 2 minutes. Stir in lemon juice and season sauce with salt and pepper.
5. Meanwhile, bring an 8-quart pot of salted water to a boil. Add potatoes and cook until tender, about 15 minutes. Drain potatoes and return to the pot with sour cream, dill, butter, salt, and pepper. Cover pan with a lid and shake until potatoes are well coated. Transfer to a large serving bowl and season with additional salt and pepper.
6. Pour sauce over cutlets and serve with dill potatoes.