Preheat an outdoor grill for high heat, and lightly oil the grate.
Place cucumbers and red peppers on grill, cook until slightly softened, about 2 minutes per side. Remove from heat.
Combine water, vinegar, sugar, kosher salt, garlic, black peppercorns, dill, and red pepper flakes in a saucepan over medium heat. Bring to a simmer and remove from heat.
Pack cucumbers and peppers evenly between two 16-ounce mason jars. Pour hot pickling liquid into jars, leaving 1/4-inch headspace. Screw on lids and let cool to room temperature. Refrigerate for 24 hours before serving.
