Cook burgers according to package instructions. Once cooked cut into thin strips. Set aside for assembly later.
Add 1 tbsp olive oil to a pot over medium high-heat.
Then add onion and spices and cook, stirring frequently, until translucent, approx. 3-5 minutes.
And garlic and cook for another 30 seconds.
Add beef stock and bring to a boil, then simmer until reduced by half, approx. 20 minutes.
Slice the buns in half, being careful not to cut all the way through.
Separate the strips of burgers into four equal portions and using a sieve dunk into the sauce for 1 minute until meat is warmed and has absorbed some juice. Repeat with the rest of the burger strips.
Strain sauce through the sieve and reserve the cooked onion.
To assemble, spread the cooked onion on the bun, add the burger strips and top with giardiniera. Serve with remaining sauce for dipping.