PIZZA DATES: Ruth Gresser is celebrating two decades of pizzamaking with a week-long numbers theme: “Everything is about the 20s,” says the Washington chef-restaurateur, who introduced her first Pizzeria Paradiso in Dupont Circle on Nov. 8, 1991, and went on to open branches in Georgetown (2002) and Alexandria (2010).

On Nov. 9, Gresser’s three pizzerias are offering all-you-can-eat pie and up to two beers per person for $20. On Nov. 11, a patron’s first brew will cost 20 cents. The next day, dining room customers will pay the same amount per topping, for up to three adornments. And come Sunday, the Old Town branch is offering a make-your-own demonstration by Gresser and giving away the first 20 pies made by children.

The secret to her signature’s longevity? Good-quality ingredients, good dough and “not too many toppings” on the crust, says Gresser, who uses fresh herbs and live, in-their-shell mussels for her pizzas, which bake to doneness in oak-fired ovens. The pizza she gravitates to most often on the standing menu is the Napoletana, sparked with anchovies and capers.

The “proudly 52” restaurateur says she is most proud of the “consistency” and “comfort” she has created in her mini-empire. “I’ve been saying to my staff for 20 years: Act like people are coming to your house and you’re the host, giving a big party.”

One of her “hosts” has been on board almost from the start. Gresser was in Paradiso’s original kitchen preparing for service 20 years ago when Scott Griswold showed up to interview for a waiter position. Their chat was interrupted by a server who called in sick.

Gresser turned to the applicant and asked, “Do you want to start right now?” Griswold’s first shift — which turned into a double when another worker couldn’t show — started “right then,” says Gresser.

Today, he’s director of operations for the brand.