Heat the oil in a medium skillet. Add the onion and sauté over medium-low heat until it is translucent.
Add the garlic and sauté for another minute before adding the green pepper. Continue to sauté until the onion is lightly golden.
Add the pinto beans along with the remaining filling ingredients and enough liquid to keep the mixture moist. Simmer, covered, for 10 minutes.
With a mashing implement, mash about half of the beans. Make sure there is enough liquid in the mixture to form a thick, saucy base. Cook, covered, for another 5 minutes.
Spoon some the bean mixture onto the centers of each flour tortilla. Sprinkle with some grated cheese, if desired, and top with a spoonful or so of salsa. Fold two sides over the filling, then roll up snugly.
Arrange 2 burritos on each dinner plate. Garnish with shredded lettuce and black olives. Pass around extra salsa. Serve at once.