Onasci began his career in the local DIT academy at age of 7, which merged with Septemvri Sofia in 2015 with Onasci moving to the Septemvri in 2017.[1] He joined first team training in 2022 at the age of 16.[2] He complete his professional debut on 17 March 2023 in a league match against Cherno More.[3] 10 days later he signed his first professional contract with the team.[1] For the 2024 season Kubrat was named a team captain for the team in the First League.[4]