ICE CREAM: Whisk together egg yolks and sugar in a medium bowl until fluffy. Meanwhile, bring milk to a light simmer in a small saucepan over medium heat; make sure milk does not scald or boil. Once it reaches a light simmer, remove milk from heat. Slowly add warmed milk to egg mixture, stirring constantly.
Return mixture to saucepan and cook over medium-low heat until thickened, about 5 minutes. Pay close attention as too much heat will cause mixture to separate and eggs to scramble. Once mixture clings to spoon, remove from heat.
Fill a large mixing bowl with ice water and place a smaller bowl inside ice bath. Add custard mixture to smaller bowl and cool for 5 minutes. Add mascarpone, lemon zest, lemon juice, and salt and mix until combined. Refrigerate mixture until chilled, about 10 minutes.
Pour custard into ice cream maker and process according to manufacturer’s instructions. For a soft-serve texture, serve immediately. For a more traditional ice cream texture, cover and freeze for at least 2 hours.
COMPOTE: Combine cherries, sugar, and ½ cup water in a large saucepan. Cook over medium heat for 10 minutes, stirring frequently.
Using a slotted spoon, transfer cherries to a bowl. Add lemon juice and salt to liquid remaining in saucepan and cook until reduced and thick enough to coat a wooden spoon, 10 to 15 minutes. Pour liquid over cherries and cool to room temperature.
OAT CRUMBLE: Heat oven to 350°F. Grease a rimmed baking sheet.
Combine oats, flour, sugar, and cinnamon in a large mixing bowl. Cut in butter until evenly distributed and mixture is shaggy. Add egg whites and oil and mix until just combined.
Pour mixture onto prepared baking sheet and spread into an even layer, then use a spatula to separate into bite-size chunks. Bake, stirring halfway through, until golden-brown, about 35 minutes. Cool to room temperature.
TO SERVE: Top scoops of ice cream with a spoonful of cherry compote and a sprinkle of oat crumble.