Prep the oven and pan. Heat the oven to 350°F (175°C).
Lightly butter a 9x13-inch baking dish or a deep pie dish if you’re baking a full crisp.
Make the fruit base. In a large bowl, toss your fruit with sugar, lemon juice, and cornstarch if needed. Spread it evenly in the baking dish.
Combine dry ingredients. In another bowl, mix oats, flour, brown sugar, granulated sugar, cinnamon, nutmeg, and salt. Break up any sugar clumps with your fingers.
Cut in the butter. Add the cold butter cubes.
Use a pastry cutter or your fingertips to rub the butter into the dry mix until it forms pea-sized crumbs. You want a sandy, clumpy texture, not a paste.
Add extras. Stir in vanilla and nuts if using. Squeeze a handful—if it holds together in loose clumps, it’s ready.
Top and bake. Scatter the topping evenly over the fruit.
Don’t pack it down; those nooks and crannies make it crisp. Bake 35–45 minutes, until the fruit is bubbly and the top is deep golden.
Rest and serve. Let it cool for at least 15 minutes. This helps the juices set so each scoop has good structure.
Serve warm with ice cream, yogurt, or whipped cream.