Prep apples: Peel, core, and slice into 1/4-inch rings. Toss with lemon juice and pat dry.
Whisk flour, 2 tbsp sugar, baking powder, 3/4 tsp cinnamon, and salt in a bowl.
In another bowl, whisk milk, beaten eggs, and vanilla until smooth.
Combine wet with dry to form a thick, pourable batter. Rest 5 minutes.
Heat oil in a wide skillet to about 350°F. Mix coating sugar and 1/2 tsp cinnamon in a shallow dish.
Dip apple rings in batter, letting excess drip. Fry in batches 1–2 minutes per side until golden.
Drain on a rack. While warm, toss in cinnamon sugar or dust with powdered sugar.