This is the weirdest thing....I did some more thinking about this. Here is what I purchased seed-wise:
Lettuce mix - of all varieties they sell (but not a salad bowl mix, just lettuce)
Pak Choi - Choy Sum
Chard - Erbette
Spinach - Oriental Giant
Chinese Kale - Gai Lohn
I originally planted lettuce in my quick start thingees, then moved some outside. I must have added some seeds to that empty space instead of just moving around lettuce (like I thought I did). I thought I'd decided not to plant any of the Choy Sum, but eventually found something growing in my lettuce that looks like Choy Sum, yellow flowers and all. (I haven't shown any pics of that yet.
Now, outside, I decided what the heck. Let's try the gai lohn anyway. Sure, it's better in the fall and it might bolt, but I can harvest it young, etc. So, I planted spinach and gai lohn outside in late April and did write down where I planted it and which type was in what space, etc. About the same time, I plant chard inside, also noting it.
So, the only thing I didn't note was the choy sum that I planted inside, but have now clearly identified.
Out of all the plants I listed above, I've planted all of them.
The spinach? Just where I put it. The gai lohn? Right next to it, looking very much like a brassica with its first true leaves. The choy sum? Where I moved it, with its yellow flowers. Chard? Also where I moved it, with it's red stems.
So, that leaves me with this mystery plant that clearly is NOT the pak choi-choy sum I planted - doesn't look anything like it, growing habit is different, size and color not the same. It's clearly not the gai lohn. It's clearly not the chard or the spinach....and, it doesn't look much like a lettuce, either. It also leaves me with yet another very young green that looks just like chard (I admit, it's only got 2 tiny true leaves, but it has red veining and crinkly leaves) yet is it NOT the Erbette I planted elsewhere. And, it's in with the lettuces! Now, maybe there is a crinkly leaf lettuce with red veining or that once it's a bit bigger will look different, but this is just weird to have two things that appear not to belong based on what I planted.
At this point, I've determined the one pictured above is clearly not something I intentionally planted. As several of you have noted, it is likely a pak choi/bok choi of some kind (thanks for the link to the Summer Jean). I suppose they could have made a mistake in my seed packet and mixed two seed types together. What points to that is that both the choy sum I can identify (but intitially thought might be lettuce because I'd forgotten I'd planted the seeds in the lettuce seed starter) and the one I couldn't identify were both mixed in with the lettuces. But, so is the chard-type green that has to be lettuce, because I'd certainly have noticed if I was planting lettuce seeds and three or four chard seeds were mixed in!
Guess I get to wait and wonder how the it grows! Susan, have you grown this variety before? Does it normally form flower buds this early? If you haven't grown it before, I guess we both (and David, too) get to find out!
I'll take pics of the chard-looking lettuce in a couple of days, too, if I haven't figured out what it is.