A sell-off in the broader stock market may still be fresh in many investors' minds, particularly those who had a portfolio weighted toward growth and tech. However, some stocks haven't just recovered from the sell-off but soared to new highs.

Two examples of companies with surging stocks in 2019 are fast-casual chain Chipotle Mexican Grill (NYSE:CMG) and customer service platform Zendesk (NYSE:ZEN). Shares of these two companies have jumped 66% and 49% year to date, respectively, fueled by strong growth in their underlying businesses.

Here's a look at their momentum.

Chipotle burrito, chips, and guacamole on a stainless-steel surface.

A Chipotle burrito. Image source: Chipotle Mexican Grill.

Chipotle Mexican Grill

Chipotle stock's surge in 2019 has been driven by the company's strong fourth- and first-quarter results.

In February, Chipotle announced that it had returned to growth in comparable restaurant transactions. Transactions were up 2% in the company's fourth quarter of 2018, driving overall comparable restaurant sales 6.1% higher. This continued in the company's first quarter; Chipotle announced late last month that its first-quarter comparable transactions were up 5.8%, helping the fast-casual restaurant deliver 9.9% comparable restaurant sales growth -- the best growth Chipotle has seen in this metric in four years.

Of course, with comparable sales growth like this, it's no surprise that revenue and operating income are rising sharply, too. Chipotle's first-quarter revenue and operating income increased 13.9% and 19% year over year, respectively.

Chipotle expects more strong growth throughout 2019. Management guided for comparable sales for the full year to rise by a mid to high single-digit percentage rate.

Zendesk

Zendesk's soaring revenue, improving non-GAAP earnings per share, and robust free cash flow have helped investors' long-term outlook for the stock.

Shares surged 15% in one day in February, following better-than-expected fourth-quarter results. The company's year-over-year quarterly revenue growth rate accelerated to an impressive 41% -- up from 38% growth in the third quarter of 2018.

Strong business growth has continued into 2019. Revenue in the company's first quarter increased 39.8% year over year to $181.5 million. Non-GAAP earnings per share doubled, rising from $0.02 in the year-ago quarter to $0.04.

Notably, Zendesk's free cash flow is rising sharply. The key free cash flow metric, which is equal to cash from operations less capital expenditures, increased 98% year over year to $36.3 million in 2018. And now management is guiding for 2019 free cash flow between $55 million and $65 million.

"Demand for our products remains strong as companies around the world, large and small, seek to transform their businesses by adopting modern software architectures and applications," said Zendesk management in the company's first-quarter shareholder letter.