Clean and chop the rhubarb. Add to a large bowl with lemon juice, sugar, and cornstarch. Toss until well coated. Pour contents into a 10 inch skillet.
In another large bowl, mix together the oats, flour, brown sugar, cinnamon, and salt. Add the cubed butter and blend well with hands until the butter is fully incorporated, but not melted. Pour the topping over the unbaked rhubarb.
Bake for 40 minutes at 350F. Remove from the oven and allow to cool for 10 minutes. Top with vanilla ice cream either in the skillet or individual dishes.
Notes
Store your rhubarb crisp tightly covered with plastic wrap or an air-tight lid in the refrigerator for 2-3 days.
You can make your crisp topping ahead of time and store in the freezer for 2-3 months. You can also freeze your chopped rhubarb for up to 1 year. However, I would recommend assembling the crisp the day of.