Combine cheese, chives, and thyme in a bowl. Gently open zucchini blossoms and fill each with a scant 1 1/2 tbsp. cheese mixture.
In a bowl, combine flour and salt. Gradually whisk in seltzer water and 1 1/2 tsp. extra-virgin olive oil until smooth. In a separate bowl, beat egg whites until stiff; fold into flour mixture.
Heat 1/2 cup olive oil in a large frying pan over medium-high heat. Dip blossoms into batter, then fry until deep golden, gently turning twice with 2 slotted spoons and reducing heat if they start to brown too quickly. Transfer to paper towels to drain.