Prep: Preheat oven to 400°F and lightly grease a 2-quart casserole dish.
Make topping: Combine sugar, flour, baking powder and salt in a medium mixing bowl. Cut in butter (just crumble it together with the flour until you have a bowl with crumbly bits), then stir in buttermilk JUST until combined - do not overmix. Dough will look scraggly and messy and sticky, that’s fine. Refrigerate for 15 minutes while assembling filling.
Make filling: Combine sugar, cornstarch, strawberry jam and lemon juice in a large mixing bowl until a thick paste forms. Add strawberries and stir well to evenly coat. Spread in prepared casserole dish.
Assemble: Remove biscuit batter from fridge. Using two tablespoons, drop about 8 biscuits on top of the strawberry filling, making sure to space them about 1 inch apart to allow space for biscuits to expand during baking. Sprinkle 1-2 tablespoons granulated sugar over biscuits.
Bake: Place casserole dish on a lined baking sheet to catch the occasional overspill. Bake for 15 minutes at 400°F, then reduce temperature to 350°F (do NOT open oven at this point!) and finish baking for 30-40 minutes, until filling is bubbly and biscuits are fully baked through. Cool on a cooling rack on the counter for at least 30 minutes before serving.
Strawberries: I prefer using fresh strawberries for this recipe, but frozen can work. Allow them to partially thaw (enough so you can cut them up), then drain them well and pat dry. I recommend adding an extra 2 tablespoons of cornstarch to the filling, as frozen strawberries tend to release more juices during baking.
Buttermilk: Feel free to use regular milk with 1 tablespoon white vinegar stirred into for a quick and easy buttermilk substitute.
Butter: Stick margarine is an acceptable substitute if you need to avoid butter for any kind of reason. Do not use a margarine spread or buttery spread, they do not work in this type of biscuit topping.
Strawberry jam: If you don’t have any on hand or don’t want to buy an entire jar just for this recipe, feel free to leave it out. Add 3 extra strawberries, 1 extra teaspoon of cornstarch and 2 extra tablespoons of sugar to the filling instead.
Cornstarch: This is my preferred thickener for a fruit cobbler, but if you don’t keep this on hand you can use all-purpose flour. The taste and texture of the filling does come out a little differently when using flour, but it does work just fine.
Do not overmix the batter for the biscuit topping. The dough really is fine looking very sticky and scraggly. That’s how the biscuits come out light and fluffy!
Do not flatten the biscuit dough after dropping it on the filling, as this will destroy the air bubbles in the dough. You want all of the air bubbles intact for a fluffy topping!
Make sure to bake the cobbler long enough. The filling and the top of the biscuits can appear cooked through after about 30 minutes of total baking time, but there may still be liquid batter underneath the baked biscuits tops. It really does need quite a long baking time. If the tops appear to get too dark after 40 minutes or so, lower the cobbler to the lower middle rack of your oven.
Allow the cobbler to cool for at least 30 minutes before serving, as the juices in the filling will thicken as they cool.
Leftovers keep covered on the counter for a day, or covered and refrigerated for up to 3 days.