Years ago, we did a test on ovens. We took water, brought it to a full boil on the fire—steam pouring out everywhere—and then threw it into a preheated oven. What happened? The steam disappeared immediately. The water stayed hot and continued boiling, but there was no visible zeiah (steam) inside the oven.

Why? Because of how modern ovens are built. In our ovens, the steam gets burned out and dissipates into the air inside the chamber. That means there’s no halachic zeiah that would transfer taste. Yes, the Gemara talks about a tanur and a kirayim, and we often translate those as “oven,” but halachah doesn’t work by simply translating terms and transplanting halachos without understanding the differences in design and function. Our ovens are fundamentally different.

So what’s the halachic bottom line?
You can technically cook meat and milk in the same oven—even at the same time—and there’s no transfer of taste or issur. Likewise, you can cook kosher and non-kosher in the same oven simultaneously, and one does not affect the other, unless there is actual physical transfer—like liquid from one dish spilling into another. So yes, keep your eyes open. If your salmon is leaking onto the food underneath it, that’s a real issue. But absent that, there’s nothing to worry about.

As for the pizza question:
The heter isn’t noten tam lifgam. Pepperoni, for example, isn’t lifgam—it’s flavorful. But that’s not the point. When you order a vegetarian slice in a pizza shop, especially in Western countries, they won’t put meat on it. It’s not about kashrus at that point—it’s about liability. No one’s going to risk a lawsuit or bad press. So if you walk in and ask, “Is this vegetarian?” and they say yes, you can rely on that—often more confidently than in a kosher store.

In short:
• Modern ovens don’t transmit taste via steam (zeiah) because it gets destroyed inside the oven.
• Therefore, halachically, you can use a non-kosher oven for kosher food as long as there is no direct contact or liquid transfer.
• And when ordering vegetarian in a non-kosher store, you can rely on what they tell you—especially when it comes to meat—not because the pepperoni is lifgam, but because it’s not going near your slice in the first place.