By growing a vegetable garden on their terraces, many Bangaloreans plan to have their meal cooked with fresh vegetables just plucked from their own terrace garden as the cost of vegetables have hit the rooftop.    In Bangalore's Hebbal. the Krishi Mela that got under way at the sprawling University of Agricultural Sciences campus on November 7 saw farmers, seed companies and nurseries answering a whole range of questions from people who want to develop terrace gardens.    Usually farmers take part in Krishi Melas in large numbers to learn about modern farming methods to increase production. But, for the first time, the Meal saw many urbanites taking part in an agricultural exhibition. Many of them purchased seeds of vegetables like tomato and beans to cultivate on terraces and in gardens. Many participants said they are interested in growing onions too on their terraces.    Some participants at the Mela said that they took up gardening after onion prices went up to Rs 80 to Rs 100.