How to get my Beagle to heel properly?

So my Beagle is nearly 2 years old. EVERY time we go outside for a walk she likes to pull and follow her nose to every little flower, patch of grass, stick, leaf, rock... or poo. I can't control her.

She knows what "heel" means. I've worked with her before and with 2 different trainers. In a closed environment she is fine. If we are walking outside in the parking lot, she's great (nothing to distract her nose)... she will walk by my side, look up at me, etc. But as soon as we get outside, if there is grass (which is everywhere), bushes, whatever, then she is uncontrollable.

I've even tried to bring treats along with me, but she completely ignores the treats... sometimes after she is calm, she will respond to treats, but only meats or cheese... which I don't know if it's good to constantly feed her leftover steak, chicken breast, hotdogs and cheese.

And it's not just pulling. Sometimes she will find some smell that makes her stop and smell for awhile. It's usually not that often, as I will tell her to "leave it" and she walks away, onto the next smell which is usually 2 inches in front of the previous spot, but there are times when I have to walk over and scoot her along.

I've tried using a harness that impedes her from walking ahead of me, but she slips out of the harness pretty easily... and I would really just like her to learn how to heel properly.