Heat a dry skillet over medium-low heat. Add the almonds, stirring constantly to make sure the almonds don't burn, and toast them until they are golden-brown and crispy. Remove the almonds and set aside.

Rinse chicken breasts and pat dry with paper towels. Make sure the chicken is all of a relatively even thickness; if they're not, place the chicken breasts between two sheets of plastic wrap and pound with a rolling pin gently just to even out the thickness.

Season chicken breasts with salt and pepper on both sides.

Heat a skillet over medium-high heat. Add olive oil and heat until shimmering, then add the chicken breasts. Don't crowd the pan (fry in batches if necessary). Cook the chicken breasts and turn after 4-5 minutes, cooking another 4-5 minutes until the juices run clear when the chicken is sliced into. Remove chicken from the plate and cover with foil to keep warm.

Reduce the heat to medium/medium low and add the butter and flour, scraping up the browned bits of chicken remaining in the pan as you swirl your spatula around. Cook this mixture for 1-2 minutes, then remove the pan from heat and add the chicken stock in a slow and steady stream, stirring vigorously to make sure you don't get any lumps.

Swirl in the mustard and return the pan to heat. Bring the mixture to a boil and then reduce the heat, allowing the mixture to reduce a little. Add the almonds and pour the entire mixture over the chicken. Serve immediately.