Juicy Bison Burger With Cucumber–Red Onion Slaw

Name: Juicy Bison Burger With Cucumber–Red Onion Slaw
Ingredients: ½ cup mayonnaise; 1 Tbsp. finely grated lemon zest; 4 Tbsp. fresh lemon juice, divided; Kosher salt; 1 large English hothouse cucumber or 4 Persian cucumbers (about 8 oz.); 1 small red onion, thinly sliced; 1 Tbsp. finely chopped mint; 1 Tbsp. finely chopped parsley; 2 tsp. white wine vinegar; 1 tsp. sugar; 1 tsp. freshly ground pepper, plus more; 1 Tbsp. granulated garlic; 1 Tbsp. smoked paprika; 2 tsp. ground cumin; 1 lb. ground bison; 4 hamburger buns, split; 1 Tbsp. extra-virgin olive oil, plus more for spatula