1. Position an oven rack in the top third of the oven and preheat to 230°C.
2. Prepare the biscuits according to the package directions, replacing half of the milk with buttermilk (this will amount to about 1/3 cup milk and 1/3 cup buttermilk for most mixes).
3. Roll or pat the dough into an approximately 8-inch round about 1/2 inch thick. Use a 2-inch round biscuit cutter to cut biscuits out of the dough. You can re-roll the scraps once and cut additional biscuits, making 14 to 16 biscuits total.
4. Transfer the cut biscuits onto an un-greased baking sheet and brush lightly with the remaining buttermilk.
5. Bake until the tops are golden brown, 6 to 8 minutes. Allow to sit at room temperature while you prepare the eggs.
6. Change the oven setting to grill. Put the jalapenos on a small baking sheet and place under the grill until charred all over, turning the jalapenos as needed, about 5 minutes.
7. Transfer to a small bowl and cover with plastic wrap. Let sit for 5 minutes, and then peel the skin away from the jalapenos. Roughly chop into small pieces, removing the seeds if you do not want the eggs too spicy. Set aside.
8. Whisk the cream, eggs and some salt and pepper together in a large bowl.
9. Heat the rapeseed oil in a 10-inch nonstick ovenproof saute pan over medium-high heat. Add the breakfast sausage and cook until browned, breaking up the meat with a wooden spoon, 4 to 6 minutes. Use a slotted spoon to remove the browned meat to a plate.
10. Reduce the heat to medium, add the onions and cook until lightly caramelized, 4 to 6 minutes. Add the chopped jalapenos and browned sausage back to the pan and stir to combine. Pour the egg mixture into the pan and cook until the eggs just start to set, about 2 minutes.
11. Sprinkle with the cheese and continue to cook until the frittata is almost set but the top is still runny, about 3 minutes.
12. Place the frittata under the grill until lightly puffed and the top is golden brown, about 3 minutes. Let stand for 5 minutes.
13. Meanwhile, turn off the grill and place the biscuits back in the oven. The residual heat of the oven will warm the biscuits through. After 5 minutes, use a rubber spatula to loosen the frittata from the pan and invert it onto a cutting board. Cut into 14 to 16 pieces (depending on how many biscuits you baked).
14. Split open the warm biscuits and sandwich a piece of frittata and some sliced green tomato between each biscuit.