Trim and discard zucchini ends. Thinly slice zucchini.
Peel onion and cut in half lengthwise, then vertically into thin slivers.
Mix zucchini, onion, and salt. Let stand 30 minutes. Rinse well and drain.
Meanwhile, stem, seed, and cut red bell peppers into thin slivers about 2 inches long.
In a 4- to 5-quart pan over high heat, bring vinegar, sugar, and pickling spice to a boil. Add bell peppers and zucchini-onion mixture. Remove from heat and mix well.
Spoon vegetables and liquid into jars, cover, and let cool. Chill at least 24 hours or up to 6 weeks.
Nutritional analysis per 1/4 cup.