I use the Spicky Salad Mix from the Lake Valley Seed Company. It has Black Simpson, Green and Red Salad Bowl, Marvel of Four Seasons, Bib, Arugula, curled cress and Mizuna. It’s a good one…. Lettuce can be sowed early and „clipped” instead of rooted out during harvest time. It will continue to grow and can be „clipped” (with scissors) every two to three weeks or so. I water my lettuce morning and night. Hopefully I’ll be eating it all summer.
We grow a lot of baby lettuce all year around, in summer we give them shadecloth, we do a cut and come again for 3 cuttings of the babies and then till that into the ground and start again in another spot. I have found bitterness to be a byproduct of either soil conditions or heat, mostly the latter- most lettuces will change flavor when they bolt. I go to a Trader Joes and look on the lettuce package ingredients for my seedling ideas, they have a few really neat mixes that all seem interesting. The one I am using now has green and red oakleaf, mizuni, romain, butterleaf, and arrugala wich I restrain as I feel this taste is heavy for me. There are at least 500 lettuces out there so sky is the limit but you do need a light soil and a lot of frequent light waterings in the heat of the day.
Try growing a few hardier leaf vedgatables that will stand up to a little heat like, red gold white and green swiss chard.
I bought some mesculin seeds last spring, threw them in a seed starting mix and watered them lightly every day. They sit on my kitchen table that gets 5-6 hours of sun through the east and southest facing windows. Every few days, I pick a few leafs for sandwiches and garnishes. Mesculin is mixed baby lettuces. Nice and sweet, not hard to grow or tend. I used a 6x6inch flat container, but you could put in in a larger pot and grow more, very quickly. Just place them in an area where they will get 5-6 hours of sunlight.