Wandering around Sofia I learned that there was a free tour to taste three Bulgarian dishes. The first is a soup called tarator, the second several kinds of toast with different sauces and some different cheese to finish.
This tour, where you do not need to sign up anywhere, is done once a day and start at 2 pm in Crystal Park, where you meet your guide and the other participants and start the tour. The guide will take you to three locations where you will taste a dish in every place, while he/she will explain you guys details and curiosities of the city in general. A joy! Finally you should give the guide a tip.
As an advice, just tell you to go before 2pm as there is a maximum of 15 vacancies and it always gets full really fast!