Your best bet is in the 1340s. Have Stefan Dusan obtain an alliance with the Venetians in exchange for naval aid to capture Constantinople. With some extra luck and determination, he manages to take the eternal city and crown himself Roman Emperor.
With this, we now have a Serb/Greek Empire that dominates around 3/4s of the Balkans, and is allied with the only other Balkan power, Bulgaria (which is in turn experiencing internal difficulties and therefore giving greater chance for unification of the Balkans under the Dusans).
Such a united Balkan state would be enough to withstand Ottoman incursions. (If anything this probably sees the Ottomans being ones attacked since Stefan was one of the first few people who took the Ottoman threat seriously)