Place chicken fillets in a pot and cover with chicken broth or water. Bring the pot to a simmer, then reduce the heat and cook the chicken fillets in the gently simmering broth for 12 to 20 minutes, until the fillets are cooked through or have reached an internal temperature of 74°C.

While the chicken is cooking, heat oil in a small saucepan over medium-high heat. Add flour and whisk for one minute.

Add the spices and mix with the roux for about 30 seconds. Slowly pour in the water while stirring, and continue whisking until no lumps remain.

Add the tomato paste and, stirring constantly, slowly bring the sauce to a simmer over medium-high heat. Remove the pot from the heat as soon as the sauce has noticeably thickened.

Remove the cooked chicken from the pot and shred the meat into small, bite-sized pieces using two forks. Add the chicken to the sauce and stir well.

Heat tortillas in the oven or microwave according to package directions. Spread some sour cream and lettuce in the center of each tortilla, top with some chicken and avocado, and finally squeeze over a little lime juice.