This fabric mod is for 1.19x and it adds a block called the Grass Plug. You will need silk touch to be able to craft this block. Its recipe is oriented vertically, as follows:

1 bonemeal
1 wheat seed
1 mycelium block

The plug will search for nearby mycelium in a 1 block radius around it, minus the diagonal blocks. It will spread slowly, like how grass spreads on dirt. The block will stop spreading and turn itself into a grass block once there is no more mycelium for that block to spread to. There is a chance for grass and tall grass to grow, as well as some flowers and saplings. Near the edge of the water, sugarcane has a chance to spawn. Natural beehives can spawn too, if you're lucky. The large mushrooms are unaffected by the grass plug, but small mushrooms will pop off since they can't be placed on the plug block.

The video shows how fast the grass plug spreads at a randomtickspeed of 100. At the end of the video, you can see the regular tick rate growth. Also, player interaction will be required if all spots on a mushroom island are to be converted, as the plug block can't reach up or down steep hills.