It's the bottom right production in a mushroom farm. You need dung (produced in rally points) and orcs* Once you have enough of those resources for a production, the blue 'diamond only' button will change to the regular green button.
*(not the Orc troops. These the little green orc heads, a resource produced in armories lvl 2+ or in orc nests/other event bldgs received after unlocking the orc chapter. These little orcs are trained in the Training Grounds to become the Orc troops. These little orcs are required for the rest of the game for bldg upgrades, encounter negotiations, etc).