This spinach mustard, Kassaundra, is it:
Mizuna
Komatsuna
Golden Frills
??
I have grown mizuna now and then and it doesn't bolt. The "Frills" from Johnny's will and yet the difference from mizuna is hard to recognize in taste or the appearance of the leaves.
Komatsuna was in my garden last fall and I've got tiny seedlings right now. It didn't bolt with a fall planting. I really wanted to try komatsuna in the spring garden. It was in a great hurry to bolt! Well, I still enjoyed it

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I don't suppose that your spinach mustard is:
Perpetual Spinach.
Chard gets the rave reviews as bolt-free and this might be a chard but, it's a little different. To me, the flavor is much more like beet greens - another chard relative. I like it just fine, bought seed for it this spring . . . then forgot to plant them

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Anyway, perpetual spinach has never bolted in my garden.
Steve