Start by heating a thick-bottomed pot or sauté pan over high heat.
Season the chicken with salt and pepper.
Add oil to the pan when it's hot.
Fry the chicken breast fillets for about 1 minute on each side. They shouldn't be fully cooked, just get a sear. Instead of frying the breasts whole, you can cut them into smaller pieces if you want more sear on them. If you don't have time to fry them beforehand, that's fine too; the chicken can cook in the sauce without being seared first.
Remove the chicken from the pan/pot and place it on a plate until needed. Turn the heat to medium and add about 10 grams of butter. Finely chop the onions and garlic and add them to the pan.
Let it sauté (become translucent and soft without coloring) for a couple of minutes. Remove from the pot and set aside for later.
Turn up the heat and add 40 grams of butter to the pot along with the curry. Let the curry "burn off" for about a minute. This enhances the flavor of the curry. Then reduce the heat to medium-high.
Add the flour and whisk it into the butter to form a roux.
Let it "bake" for 2-3 minutes to eliminate the raw flour taste.
Gradually add the chicken stock while whisking vigorously to avoid lumps. Using warm stock helps bind the sauce and prevents clumping.
Continue until all the stock is used. Then add the whole milk and season with salt and pepper.
Add the cream and let it simmer for a few minutes.
Cut the chicken into smaller cubes and add them to the sauce along with the sautéed onions and garlic.
Reduce the heat to low and let it simmer for 5-7 minutes, depending on the size of the chicken pieces.
Season to taste with salt, pepper, and possibly more mango chutney. If you don't have mango chutney, you can use vinegar and sugar instead.