You may substitute most of the ingredients if wished and make different versions:
ITALIAN - Spread the base with sundried tomato pesto, toss the vegetables with 1 tsp. dried oregano then proceed. You may used mozzarella if wished.
MEXICAN - Mix 2 tbsp. tomato puree with 2 tbsp. water. Add 1 tbsp. finely chopped coriander and a pinch of dried chillies. Spread this over the base. Add 200g tinned kidney beans, to the vegetable mixture, omitting the meat if wished, then proceed as above. .
GREEK - Mix 2 tbsp. tomato puree with 2 tbsp. water, 1/2 tsp. crushed garlic/garlic and 1/2 tsp dried oregano. Spread over the base. Add 100 chopped olives and 100g chopped feta to the vegetable mixture and proceed as above, omitting the grated cheese.
You may also swap the ham for chicken, chorizo or tuna