In a large skillet, saute the artichokes, pepper and onion in oil until tender; stir in cheese. Remove from the heat.
In a food processor, combine mayonnaise and tomatoes; cover and process until finely chopped.
Spread four bread slices with half of the mayonnaise mixture; layer with a spinach leaf, artichoke mixture and remaining spinach. Spread remaining bread with mayonnaise mixture; place on top. Butter outsides of sandwiches.
On a griddle, toast sandwiches for 2-3 minutes on each side or until bread is lightly browned.