Make the crust. Heat oven to 350ºF. Place the flour, baking soda, salt, and butter in a food processor fitted with a knife blade. Pulse until the butter is cut into small bits. With the motor running, slowly add the milk through the feed tube until the dough comes together and sits on the blade. Remove and divide in half. On a lightly floured counter, roll half the dough into an 11-inch round. Use it to line a 9-inch pie plate.
Fill it. Layer the tomato slices, basil, and scallions in the pie shell. Top with half the Cheddar. In a small cup, combine the mayonnaise, lemon juice, salt, and a few grinds of pepper. Pour over the tomatoes and top with the remaining Cheddar.
Cover it. Roll out the remaining dough and place it on top of the filling, folding the edge under and crimping it to seal. Cut 3 or 4 slits in the dough so the steam can escape. Bake for 45 to 50 minutes or until bubbly. (Cover lightly with foil if the crust begins to get too brown.) Remove from oven. Cool 10 minutes before cutting into wedges.