Rinse the chicken fillet and pat dry. Dice very finely.
Heat oil in a pan over medium-high heat. Add the diced chicken and fry for 5-7 minutes, stirring occasionally, until starting to brown.
Add minced garlic and sauté for 1-2 minutes until fragrant and translucent.
Dice the red pepper. Add to the pan along with the peeled and chopped tomatoes. Stir in ½ tablespoon chili powder and ½ teaspoon cumin. Season with salt and pepper.
Reduce heat to medium, cover the pan, and simmer for 15-20 minutes until the chicken is cooked through (internal temperature 74°C / 165°F).
While the chicken simmers, make the guacamole: halve the ripe avocados, remove the pit, and scoop the flesh into a bowl. Chop coarsely.
Add 2 tablespoons lemon juice and mash with a fork until mostly smooth with a uniform texture.
Stir in 2 tablespoons sour cream and 1 minced garlic clove. Season with salt and pepper to taste.
If using homemade pancakes, heat oil in a separate pan over medium-high heat and fry thin pancakes until lightly golden on both sides. If using ready-made tortillas, skip this step.
Lay a tortilla or pancake on a work surface. Spoon chicken mixture down the center, roll tightly into a burrito, and place seam-side down in a baking dish. Repeat with remaining tortillas.
Sprinkle the grated cheese over the burritos.
Preheat the oven to 200°C (392°F) for electric ovens, gas mark 3 for gas ovens, or 180°C (356°F) for convection ovens.
Bake for 5 minutes if the burritos are still warm, or 10-12 minutes if prepared ahead and refrigerated.
Serve the burritos with a side of iceberg lettuce, tomatoes, peppers, and the guacamole.
Variation: spread the guacamole over the burrito tops before sprinkling with cheese in step 11. The guacamole will bake with the cheese for a gratin effect.