Chives? Parsley? Bulls Blood beets? Sorrel? It's a potager so edibles would make sense.

nasturtium....showy flower & edible

I used parsley for edging this year and loved it. Also Lobelia and petunias, both of which will hang down the sides and add great color.

If you go for Nasturtiums choose a dwarf variety. A single plant of a traditional creeping nasturtium would fill that bed. Plus if the soil is rich, as it would be for vegetables, Nasturtiums would grow enormous and leafy.

What works for me is to use different biennial or perennial edgings combos in each bed so I can remember what is planted where and direct a family member to go out to get more of something while I'm in the kitchen.

I plant curly parsley with garlic chives and flat leaf parsley with onion chives. I let flat leaf parsley reseed & move seedlings to wear I want them or leave as a groundcover for brassicas. I am not letting curly parsley reseed this 2nd year. What we don't eat I feed to the our rabbits & chickens, so it's used up.

To tie the beds together I've planted Calendula, pot marigold and a few pansies. I also sometimes add a few bedding or dwarf dahlias in summer. Slugs are attracted to the pansies & the dahlia so eat them before my lettuces.

Small marigolds, alyssum, calendula, small zinnias and pansies/violas, if there's a little shade :)

Here's some info on Boxwood Basil. It stays small and compact, 8" to 14" tall and wide. I might try this myself!

Here is a link that might be useful: Boxwood Basil