Cut the fillets into medallions and season with black pepper and salt. Fry with olive oil in a very hot skillet (always fry 2 medallions so you don't lose the temperature of the skillet). Be careful not to burn the crust left in the skillet as it will be important to finish the broth.
Remove the fillets and in the same skillet add a little more oil and fry 1/2 onion in small cubes. The onion will release water and cause that crust that was in the skillet to drop into the sauce. Add 1 cup of beef stock (preferably homemade) and let it reduce by half.
When reduced, strain this broth to remove the onion and return the broth to the skillet. Add the green peppers (to taste - I use 25g) and then the cream. Let it thicken a little and adjust the salt. Return the fillets to the sauce and let it heat up a bit if necessary.
Cook the baroa potatoes in water and salt.
When they are very soft, drain and mash the potatoes.
Add (while still hot) the milk and butter. (If you want a softer puree, add more milk). Stir with a fouet to give the creaminess.
Finish with nutmeg and salt to taste.