Your easiest option is to eat at the cafeteria on the new campus for lunch because they are usually out of food by 2pm and they are closed outside of this time frame. Bear in mind that the cafeteria works only on weekdays!
The closest restaurant is Avocado and it is located at the Plaza Hotel opposite the new campus. Other restaurants can be found about 1km away, for example Yovana, Hitar Petar or Pileto.
Other options are finding food at the nearby supermarkets. The nearest one is Lidl – 1.4km away which is a nice 20-minute walk. Opposite the Lidl, there is a small fast food called Go Grill which serves Bulgarian BBQ. Two other supermarkets after Lidl are Leksi and Billa, 2km away, a nice 30-minute walk. There is one section, where you can find already prepared meals.
If you explore the city a bit further, you can find many local bakeries. You could try savory pastries like banitsa or sweet pastries with chocolate or jam.
If you don’t feel like going anywhere, you can order food to your dorms from delivery companies like Glovo or Takeaway. They offer many varieties of meals.