Of course, a tarte tatin can never be very keto. But with a few changes to the original recipe it will be quite a bit lower on carbs. 

So, first make the dough for the sponge. Normally, you would start out with equal amounts of flour, sugar and eggs (200 gr each). I replace regular flour by the same quantity of almond flour and the sugar by about 150 gr of erytritol, a natural sweetener. Add half a teaspoon of baking soda and a teaspoon of baking powder. Mix well and set apart.

You could do tarte tatin in a ceramic bowl, or use an oven-proof skillet. The last is my preferred method.

Peel the pears and slice in half lengthwise. Remove the core.

Put some butter in the skillet and put the pear halves in with the flat side down. Wait until the pears start caramelizing, helping them with a bit of sugar (substitute) if necessary.

Take the skillet off the heat and let the contents cool down a little bit.  Grease the side of the skillet with some butter. Pour the dough over it, making sure the top is neatly flat.

Pre-heat your oven at 170° Celsius (340° Fahrenheit). Place the skillet in the middle of the oven and bake for 35 to 40 minutes. Take the skillet out of the oven and test if the dough is cooked through.

If so, let cool for a few minutes. Then comes the most adventurous part of the process. Place a plate (roughly the size of the skillet) on top and hold, and in one smooth movement turn over. You did it!